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Small QRP antenna

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Fred Moore-2

Re: Small QRP antenna

oops here I am typing 1/4 when the whole time I was thinking 1/2..
thanks for correcting me.. Fred

On 12/29/16 11:09 AM, K9MA wrote:

> I think Fred meant an end fed 1/2 wave, NOT 1/4 wave.
>
> 73,
>
> Scott K9MA
>
> ----------
>
> Scott Ellington
>
> --- via iPad
>
>> On Dec 29, 2016, at 9:30 AM, Fred Moore <[hidden email]> wrote:
>>
>> With an end fed 1/4 wave you only need a counterpoise that is about .05
>> wave length. it can be the coax or another counterpoise. Although a
>> counterpoise has some effect on radiation pattern of a quarter wave
>> radiator, it is minimal. You won't see much improvement in performance
>> by going above .05 wave length.. it's not worth the trouble to increase
>> the counterpoise length.. Fred
>>
>>
>>> On 12/29/16 9:01 AM, mjisted wrote:
>>> Looking at your text message on the elecraft board. About a half wave
>>> length ...here is a question for you. I'm going to put up a half
>>> wavelength of wire for the 5 mhz band. ..hyendfed can I use a quarter
>>> wave length of counter point as a radiator ground plane. ..
>>> The. ..G0MVP.
>>>
>>> [hidden email]
>>>
>>>
>>> Sent from Samsung Mobile
>>>
>>>
>>> -------- Original message --------
>>> From: Fred Moore
>>> Date:29/12/2016 13:04 (GMT+00:00)
>>> To: [hidden email]
>>> Subject: Re: [Elecraft] Small QRP antenna
>>>
>>> Don,
>>>
>>> Thank you very much, for pointing out a very common issue that gets
>>> mixed up into discussions. Your point is so valid the random wire
>>> always seems to get mixed into the 1/2 wave discussion for some
>>> reason.. and should never be mixed together. you are absolutely
>>> correct.. thanks again regards.. Fred
>>>
>>>
>>>> On 12/29/16 12:33 AM, Don Wilhelm wrote:
>>>> Scott,
>>>>
>>>> If the radiator is not a half wave long, it is not a "half wave".
>>>> Yes, a random length wire may work, but please do not call a random
>>>> length a half wave - the length of a half wave is quite specific.
>>>> Similarly, a quarter wavelength is a specific length at a particular
>>>> frequency.
>>>>
>>>> 73,
>>>> Don W3FPR
>>>>
>>>>> On 12/28/2016 11:20 PM, K9MA wrote:
>>>>> The counterpoise for an end fed half wave (or multiple) really doesn't
>>>>> have to be anywhere near a quarter wave long.
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>>> --
>>> Fred Moore
>>> email: [hidden email]
>>> [hidden email]
>>> phone: 321-217-8699
>>>
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>> --
>> Fred Moore
>> email: [hidden email]
>> [hidden email]
>> phone: 321-217-8699
>>
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--
Fred Moore
email: [hidden email]
[hidden email]
phone: 321-217-8699

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Fred Moore-2

Re: Small QRP antenna

In reply to this post by Jim Brown-10

Jim, almost everyone on the list immediately noticed that I
accidentally typed 1/4 when I actually meant to type 1/2. They most
likely noticed it because the thread has been almost 100% about 1/2 end
fed radiators. The person I replied to specifically asked about adding
a 1/4 wave counterpoise to a EFHW antenna. It appears that you deleted
the part of the message I was replying to and only kept the portion you
wanted everyone to see..

My response (this post) has nothing to do with my miss-type (we all oops
sometimes) it has 100% to do with making sure you have not confused
anyone on the list into thinking an EFHW is the same as a 1/4 wave
radiator..

BTW I 100% agree with everything you said about 1/4 wave radiators.. I
absolutely do not agree that ground losses are the same on 1/2 wave
radiators as they are on 1/4 wave radiators.. Discussions about current
node locations can be had another day..

Keep spreading accurate knowledge, everyone needs it, just make sure the
knowledge you spread applies to the subject.. otherwise it creates even
more confusion..

Regards.. Fred

On 12/29/16 1:58 PM, Jim Brown wrote:

> Nothing in this post is true. Optimum length for a counterpoise is a
> quarter wave. Radials are a form of counterpoise, and their optimum
> length is also a quarter wave. More radials reduce ground losses.
> Failing to provide an adequate counterpoise will cause the antenna's
> return current to flow on your coax, which makes the rig hot with RF.
> That's not much of a problem with 5W, but it can be a very big deal
> with 100W or more.
>
> Of course, it does depend on what you mean by "much" and "minimal."
> Inadequate counterpoise or radial systems can easily reduce you signal
> by 6 dB. That's an S-unit, and it reduces your 5W signal to 1.25W. I
> wouldn't call that minimal or "not much."
>
> 73, Jim K9YC
>
> On Thu,12/29/2016 7:30 AM, Fred Moore wrote:
>> With an end fed 1/4 wave you only need a counterpoise that is about .05
>> wave length. it can be the coax or another counterpoise. Although a
>> counterpoise has some effect on radiation pattern of a quarter wave
>> radiator, it is minimal. You won't see much improvement in performance
>> by going above .05 wave length.. it's not worth the trouble to increase
>> the counterpoise length..
>
>
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Jim Brown-10

Re: Small QRP antenna

Sorry Fred, I replied to what you wrote, not what you thought. Sometimes
my mind-reading skills fail me. :) I agree with your comments in the
context of an end-fed half wave radiator. I modeled this in NEC several
years ago. The result suggested a dB or so for a full radial system with
a half wave vertical over poor ground.

73, Jim K9YC

On Thu,12/29/2016 12:50 PM, Fred Moore wrote:
> Jim, almost everyone on the list immediately noticed that I
> accidentally typed 1/4 when I actually meant to type 1/2.

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k6dgw

Re: Small QRP antenna

In reply to this post by Fred Moore-2

I'm fairly certain that Fred meant 1/2 wave ... correct hand, wrong
finger, happens to me all the time.

This has been going on for a long time, here's a summary and maybe it
can rest for awhile:

1. Elecraft ATU's are L-networks. Theoretically, one of the 4 L-net
configurations will match any impedance to any other impedance.
Practically, for some attempted matches, the required values of L and/or
C become unattainable.

2. Any conductor you can get power into will radiate.

3. "Random" length wires will exhibit random impedances at the end [or
anywhere else along them]. ATU's will *generally* match those
impedances to 50 ohms [or close] for you.

4. Some "random" lengths are better than others for matching with an
L-network. They all radiate however [see #2]. 41-43 ft is one, ~26 ft
is another. Random length conductors require a counterpoise of some sort.

5. All half-wavelength conductors are created equal in terms of
current/voltage distribution. The ends are open circuits ... zero
current/infinite voltage. The center is a short circuit ... infinite
current/zero voltage. The power on the conductor is constant [ignoring
wire resistance]. It doesn't matter where you feed it. A full-wave
wire is just two half-waves fed in series.

6. Infinities aren't real of course, the ends are high voltage/small
current = Hi-Z [several K ohms]. The center is high current/small
voltage = Lo-Z [40-70 ohms].

7. Feeding a half-wave at the end would seem to require that there be
"something" for the other side of the circuit so current can flow.
Practically, because the impedance is very high, the current is very low
and the "something" doesn't have to be very much. In fact, if the
primary of the matching transformer is fed with a couple feet of coax,
the shield will provide that "something." So will the wiring around you
[headphone cable, power cable, key cable], and you if you're holding
your radio.

8. For NEC-2 models, connecting a source to the end of the half-wave
wire means there is no return path and it essentially tries to divide by
zero, maxing out a floating-point value. It's a little more complex
than that, but that's the essence. In such a model, you need to provide
a return path for a very small current. On 20 meters, one of my field
half-waves requires about 6 in. Anything longer makes no difference.

One easy way to construct a "random" length antenna is to peel the outer
cover from a length of RG-58, separate the braid at the bottom and pull
the center conductor through the hole. The empty braid then becomes the
counterpoise and you can trim it to find the "sweet spot" your ATU can
easily match.

Hope this helps on the subject. Happy New Year to all

Fred K6DGW
Sparks NV DM09dn
Washoe County

--Northern California Contest Club
--CU in the Cal QSO Party
--7-8 Oct 2017

On 12/29/2016 7:30 AM, Fred Moore wrote:

> With an end fed 1/4 wave you only need a counterpoise that is about .05
> wave length. it can be the coax or another counterpoise. Although a
> counterpoise has some effect on radiation pattern of a quarter wave
> radiator, it is minimal. You won't see much improvement in performance
> by going above .05 wave length.. it's not worth the trouble to increase
> the counterpoise length.. Fred
>
>
> On 12/29/16 9:01 AM, mjisted wrote:
>> Looking at your text message on the elecraft board. About a half wave
>> length ...here is a question for you. I'm going to put up a half
>> wavelength of wire for the 5 mhz band. ..hyendfed can I use a quarter
>> wave length of counter point as a radiator ground plane. ..
>> The. ..G0MVP.
>>
>> [hidden email]
>>

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Bill Frantz

Re: Small QRP antenna

In reply to this post by Don Wilhelm

I may be seriously wrong, but shorter than 1/4 wave ground
mounted radials are probably affected by the same ground tuning
effects that allow a Beverage on the Ground antenna to be
considerably shorter than one mounted above the ground. If this
is really what is going on, then a bit longer than 1/8 wave
should be about right for radials on/under the ground.

73 Bill AE6JV

On 12/29/16 at 9:05 AM, [hidden email] (Don Wilhelm) wrote:

>With buried radials, they do not have to be 1/4 wave long, and
>a single one will probably not make much difference over your
>ground rod.
>What you want to accomplish is a "ground screen", which means
>many short (about 1/8 wavelength) close to the base of the
>radiator and a few extending out further (to 1/4 or 1/2 wavelength).
>
>Ground conductivity plays a great part in buried radials, and
>what you are trying to accomplish is to increase the ground
>conductivity in the area at the base of the antenna.
>
>If you were instead using elevated radials, they would have to
>be resonant - think of the various "ground plane" antennas that
>have been published - 1/4 wave radiator plus 2 or 4 1/4 wave radials.
>Buried radials do not have to be resonant, elevated radials
>must be resonant. The ground effects will alter the resonance
>of buried radials while radials in the air are a part of the
>total antenna resonance.

-----------------------------------------------------------------------
Bill Frantz |Security, like correctness, is| Periwinkle
(408)356-8506 |not an add-on feature. - Attr-| 16345
Englewood Ave
www.pwpconsult.com |ibuted to Andrew Tanenbaum | Los Gatos,
CA 95032

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k6dgw

Re: Small QRP antenna

In reply to this post by Richard Fjeld-2

Ground rods are pretty ineffective at RF, Dick although I'm not
surprised your Inv-L works good. It all depends on the quality of the
"earth" around your antenna. If yours is good, you're fortunate.
Saltwater is great. Keep in mind that you're trying to provide a return
for an electromagnetic field which is extended in space. The lower you
can make the resistance of that return, the better.

If your radial field is elevated off the ground, they need to be 1/4
wave [or so] long. One will make a big difference over none. Two in
opposite directions will be better but not by as much. Additional ones
will help but to a decreasing degree.

If your radial field is on the ground or buried, length doesn't really
matter, nor do they all have to be the same length, straight, or evenly
spaced. All you're really doing with buried conductors is enhancing the
conductivity of the "earth" return path over the space of the field.

Fred K6DGW
Sparks NV DM09dn
Washoe County

--Northern California Contest Club
--CU in the Cal QSO Party
--7-8 Oct 2017

On 12/29/2016 8:49 AM, Richard Fjeld wrote:

> With reference to below, I have a question please. I put up a quarter
> wave inverted-L for 160 meters. My only counterpoise is the ground rods
> at the base of the tower. I wasn't expecting much, but it does so well
> I'd like to add a decent counterpoise.
>
> If I bury a quarter wave wire, would it be best to put it under the
> Ariel wire, or opposite from it?
> Thanks in advance,
> Dick, n0ce
>

Jim Brown-10

Re: Small QRP antenna

In reply to this post by Bill Frantz

On Thu,12/29/2016 1:53 PM, Bill Frantz wrote:
> If this is really what is going on, then a bit longer than 1/8 wave
> should be about right for radials on/under the ground.

I did an NEC study several years ago placing a half wave dipole at
heights above ground from several feet down to an inch, and varied the
length of the dipole so that it remained resonant at each height. From
that I plotted VF. As close to the earth as I could model it, VF was
about 0.7. At heights of 3 ft or more, VF on 160M was close to 1.

N6LF published a detailed study of radial lengths and recommended an
optimum length for elevated radials slightly less than a quarter wave.
His basis was that making them slightly shorter caused current
distribution between them to be more equal, which reduces ground losses.
Rudy's website is a wealth of great info about radials.

73, Jim K9YC

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Don Wilhelm

Re: Small QRP antenna

In reply to this post by k6dgw

Fred,

You summed up antennas in general very well, and especially end fed dipoles.

I would like to add a bit to your item 7 - dealing with the matching of
a half wave end fed to a 50 ohm line - and the counterpoise wire.

There are two common ways of matching to an end fed half wave. One is
by using a tuned resonant circuit with a link (or tap) to the coax. In
the case of a link, the short counterpoise needs to be connected to the
'bottom' end of the resonant circuit. If the link is isolated from the
winding of the resonant circuit, the coax shield cannot provide that
function, the counterpoise must be a separate wire.
If the matching network is the commonly used 9:1 unun, then the bottom
of the high impedance side is connected to the coax shield, and the coax
shield can serve as the counterpoise wire. That is why one needs to use
a short length of coax with those antennas and not connect directly to
the rig.

Most commercial end fed matching networks can use the coax shield and no
counterpoise wire is necessary, but you must use a short length of coax.

73,
Don W3FPR

On 12/29/2016 4:49 PM, Fred Jensen wrote:

> 7. Feeding a half-wave at the end would seem to require that there be
> "something" for the other side of the circuit so current can flow.
> Practically, because the impedance is very high, the current is very low
> and the "something" doesn't have to be very much. In fact, if the
> primary of the matching transformer is fed with a couple feet of coax,
> the shield will provide that "something." So will the wiring around you
> [headphone cable, power cable, key cable], and you if you're holding
> your radio.
>
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Wes Stewart-2

Re: Small QRP antenna

In reply to this post by Jim Brown-10

Except for what I'm about to say, what Jim says in his first sentence is true,
which of course includes his post:-)

The velocity of propagation in a wire, near to, on, or below the ground is
different from the same wire in free space. How different? Most people don't
know, don't care or couldn't determine it if they wanted to. But it has been
shown that for these radials, especially when few in number, efficiency actually
/improves /when their lengths are less than 1/4 lambda. See: "Experimental
Determination of Ground System Performance for HF Verticals...", by Rudy
Severns,* N6LF, QEX, Jan/Feb 2009 pp 48-52.

* I can't recommend highly enough Rudy's work on vertical antennas.
http://rudys.typepad.com/ He writes this stuff faster than I can read it.

Wes N7WS

On 12/29/2016 11:58 AM, Jim Brown wrote:

> Nothing in this post is true. Optimum length for a counterpoise is a quarter
> wave. Radials are a form of counterpoise, and their optimum length is also a
> quarter wave. More radials reduce ground losses. Failing to provide an
> adequate counterpoise will cause the antenna's return current to flow on your
> coax, which makes the rig hot with RF. That's not much of a problem with 5W,
> but it can be a very big deal with 100W or more.
>
> Of course, it does depend on what you mean by "much" and "minimal." Inadequate
> counterpoise or radial systems can easily reduce you signal by 6 dB. That's an
> S-unit, and it reduces your 5W signal to 1.25W. I wouldn't call that minimal
> or "not much."
>
> 73, Jim K9YC
>
> On Thu,12/29/2016 7:30 AM, Fred Moore wrote:
>> With an end fed 1/4 wave you only need a counterpoise that is about .05
>> wave length. it can be the coax or another counterpoise. Although a
>> counterpoise has some effect on radiation pattern of a quarter wave
>> radiator, it is minimal. You won't see much improvement in performance
>> by going above .05 wave length.. it's not worth the trouble to increase
>> the counterpoise length..

Bill K9YEQ

Re: Small QRP antenna

In reply to this post by Mike Morrow-3

Mike,
Over the years I have come to know Don as always succinct and to the point.
AND, he does know his stuff nearly 100% or he will not comment. He will not
waste his time responding to this accolade. We go back to the K2 field test
and he was much a pleasure to work with then as now. Oh, I and I think his
knowledge and writing skill has improved with age!

73,
Bill
K9YEQ

-----Original Message-----
From: Elecraft [mailto:[hidden email]] On Behalf Of Mike
Morrow
Sent: Thursday, December 29, 2016 12:09 PM
To: [hidden email]
Subject: Re: [Elecraft] Small QRP antenna

Don wrote:

> A 1/2 wave radiator fed at the end theoretically does not need any
> counterpoise, but in practice it does - but only needs to be about
> 0.05 wavelength long.
>
> Refer to some antenna books (the ARRL has a good one) or the ARRL
> Handbook for good information about basic antennas.

The concise clarity of Don's input to this thread is refreshing. Such seem
to occur rarely in ham discussions about antennas.

With respect to classical end-fed half-wave antennas, I think of the end-fed
Zepp[elin] and the common VHF J-poles.

Mike / KK5F
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Bill K9YEQ

Re: Small QRP antenna

In reply to this post by Jim Brown-10

Having "only" 5 watts doesn't change the need for proper termination and
antenna match. 5 Watts, or any wattage, for that matter can be a problem.
5 watts MAY make the problem less physically an problem.

73,
Bill
K9YEQ

-----Original Message-----
From: Elecraft [mailto:[hidden email]] On Behalf Of Jim
Brown
Sent: Thursday, December 29, 2016 12:59 PM
To: [hidden email]
Subject: Re: [Elecraft] Small QRP antenna

Nothing in this post is true. Optimum length for a counterpoise is a quarter
wave. Radials are a form of counterpoise, and their optimum length is also a
quarter wave. More radials reduce ground losses.
Failing to provide an adequate counterpoise will cause the antenna's return
current to flow on your coax, which makes the rig hot with RF.
That's not much of a problem with 5W, but it can be a very big deal with
100W or more.

Of course, it does depend on what you mean by "much" and "minimal."
Inadequate counterpoise or radial systems can easily reduce you signal by 6
dB. That's an S-unit, and it reduces your 5W signal to 1.25W. I wouldn't
call that minimal or "not much."

73, Jim K9YC

On Thu,12/29/2016 7:30 AM, Fred Moore wrote:
> With an end fed 1/4 wave you only need a counterpoise that is about
> .05 wave length. it can be the coax or another counterpoise.
> Although a counterpoise has some effect on radiation pattern of a quarter
wave
> radiator, it is minimal. You won't see much improvement in performance
> by going above .05 wave length.. it's not worth the trouble to
> increase the counterpoise length..

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Edward R Cole

Re: Small QRP antenna

In reply to this post by Rick Dwight

I've stayed out of this discussion so far.

I can comment on building a J-pole for 10m and 6m. I used a 24-foot
tall 1-inch diameter aluminum mast held by a 8-foot section of
Rohn-25G "planted" in the ground (no concrete). I used 1/4 wave stub
space 2-inch running parallel to the mast from ground level on 10m.
the bottom ends were strapped together and coax was run up about
6-inches above the ground end with center tied to stub and shield to
mast. Good 50-ohm match over about 28.300-28.700 MHz and under 2:1
over a MHz bandwidth. I did not ground the J-pole other than that
provided by the short tower.

I added a 6m J-pole to the top section running a separate coax feed
and it had little effect on tuning the 10m antenna. I tuned it for
52-MHz FM but found the only repeater was at marginal distance with
no local activity so its not in current use (the 24-foot pole acts as
end support for my 80/40m inverted-V with long nylon cord so the two
antenna do not interact).

I did make a coax coil RF choke at base of the antenna to inhibit
common-mode radiation and coax was buried a couple inches deep for
about 50-feet run to the house. I always intended to add buried
radials but never got around to it.

I had this up as a web-page until my site was hacked a few years ago
and never restored it.

I also have a very-short inverted-L on 600m: 43-foot by 122-foot
which is currently down for repair. My radials were lain on ground
and at 70-foot not anywhere near 1/4 wave (465-foot). Ground
resistance is 20-ohms measured whereas radiation resistance is 0.83
ohm per EZNEC5. Obviously not efficient, but definitely QRP when
running 100w (less than a watt radiated). Worms are cozy! SWR
bandwidth is 5-KHz.

Of course neither are QRP portable antennas. One could make the 10m
version from ladder line plus 1/2 wave top wire either as a zepp or J-pole.
I think the zepp configuration would be nice for quick portable use
if you have two vertical supports (one could be an 8-foot aluminum
pole with standoff rod for the 1/4 wave section). I might give that
a look for use with my trailer in a campground while RVing.

73, Ed - KL7UW
http://www.kl7uw.com
Dubus-NA Business mail:
[hidden email]

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Don Wilhelm

Re: Small QRP antenna

In reply to this post by Jim Brown-10

Walt,

I differ, antenna wires do have a velocity factor to consider. I built
a 6 meter Moxon beam for my grandson out of insulated wire, and the
resonant frequency was considerably low. Stripping the insulation from
the wire brought its resonance point up to what was expected.
The only reason was because the velocity factor for the insulated wire
was less than that of non-insulated wire.

73,
Don W3FPR

On 12/29/2016 6:33 PM, Walter Underwood wrote:

Wes Stewart-2

Re: Small QRP antenna

In reply to this post by Jim Brown-10

Not so.

On 12/29/2016 4:33 PM, Walter Underwood wrote:
> This is quite likely overly pedantic, but “velocity factor” is a characteristic of transmission lines. Interestingly, it is independent of frequency (up to the limit of the dielectric).

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David Gilbert

Re: Small QRP antenna

In reply to this post by Jim Brown-10

You aren't being overly pedantic ... you're simply being wrong. Velocity
factor for electromagnetic emmissions is the ratio of any signal in any
environment compared to its value in free space. The velocity factor of
a signal in a transmission line is a function of geometry and
surrounding materials, and the same is true of a signal in a single wire.

https://en.wikipedia.org/wiki/Velocity_factor if you aren't convinced.

Dave AB7E

On 12/29/2016 4:33 PM, Walter Underwood wrote:

> This is quite likely overly pedantic, but “velocity factor” is a characteristic of transmission lines. Interestingly, it is independent of frequency (up to the limit of the dielectric). It depends on the geometry of the line and the dielectric material.
>
> Antennas don’t have a velocity factor. The shortened elements are caused by capacitive loading against (RF) ground. There is a percentage of the free-space electrical length due to capacitive loading, but it is not a velocity factor. I don’t think this has a snappy shorthand other than “electrical lengthening due to capacitive loading”.
>
> For example, dipoles with capacity caps on the ends, like the N6BT designs, don’t change the velocity of propagation along the elements. They use massive capacity loading on the ends (the high-voltage part of the dipole) to shorten the elements.
>
> http://www.force12inc.com/products/sigma-20-hd-20-meter-heavy-duty-vertical-dipole.html <http://www.force12inc.com/products/sigma-20-hd-20-meter-heavy-duty-vertical-dipole.html>
>
> OK, overly pedantic mode off, plus I’ve nearly hit the limit of what I remember from my fields and waves class decades ago. I was pretty happy to get a B- in that class.
>
> wunder
> K6WRU
> Walter Underwood
> CM87wj
> http://observer.wunderwood.org/ (my blog)
>
>> On Dec 29, 2016, at 2:08 PM, Jim Brown <[hidden email]> wrote:
>>
>> On Thu,12/29/2016 1:53 PM, Bill Frantz wrote:
>>> If this is really what is going on, then a bit longer than 1/8 wave should be about right for radials on/under the ground.
>> I did an NEC study several years ago placing a half wave dipole at heights above ground from several feet down to an inch, and varied the length of the dipole so that it remained resonant at each height. From that I plotted VF. As close to the earth as I could model it, VF was about 0.7. At heights of 3 ft or more, VF on 160M was close to 1.
>>
>> N6LF published a detailed study of radial lengths and recommended an optimum length for elevated radials slightly less than a quarter wave. His basis was that making them slightly shorter caused current distribution between them to be more equal, which reduces ground losses. Rudy's website is a wealth of great info about radials.
>>
>> 73, Jim K9YC
>>
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Jim Brown-10

Re: Small QRP antenna

In reply to this post by Bill K9YEQ

On Thu,12/29/2016 3:07 PM, Bill Johnson wrote:
> Having "only" 5 watts doesn't change the need for proper termination and
> antenna match.

By proper termination, I think you're thinking about SWR and a match to
the transmitter. This thread is several days and 20+ posts old, but I
think I remember that the intent of the original poster was for portable
operation. But I could be wrong. :)

There are two reasons for low SWR and "matching." One is to get the
transmitter to provide power to the transmission line. If the mismatch
is too high, the transmitter will "fold back" (reduce its drive to
reduce its output power) to protect itself. Providing that match is the
function of an "antenna tuner."

The second reason is to reduce loss in the line due to a mismatch. The
SWR in a transmission line is determined entirely by the match between
the antenna and the line. It is NOTHING to do with the match between the
transmitter and the line. In typical QRP portable setups, transmission
lines are rarely long enough for loss to matter, so the match between
the antenna and the line usually doesn't matter. And the antenna tuner
takes care of match at the transmitter so that the output stage can
supply its full power.

73, Jim K9YC

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Jim Allen

Re: Small QRP antenna

In reply to this post by Rick Dwight

My experience matches this precisely.

~35 years ago, I built the huge quad described in "Evolution of a Quad Array" from an article in QST, May 1978, IIRC. It was done with help from experienced antenna builders, measured very meticulously. When we finally got it up on the tower, it was awful, weird SWRs, no F/B, nothing seemed right. I checked all the feedlines and connectors. Nothing amiss.

I had found a roll of No. 12 insulated wire at a surplus place and used that for all the elements. In discussions with the owner of the quad in the article and the designer, we concluded the problem was the insulation, as the design used enameled No. 12, different velocity factor. I spent the next ~2 weeks of evenings, until 10-11 at night, with a pocket knife on my roof scraping insulation from ~1200' of wire in situ. Not only was it no fun, the neighbors never looked at me the same again. Once that was done, the antenna performed wonderfully, as expected.
73 Jim Allen W6OGC
> Walt, I differ, antenna wires do have a velocity factor to consider. I built a 6 meter Moxon beam for my grandson out of insulated wire, and the resonant frequency was considerably low. Stripping the insulation from the wire brought its resonance point up to what was expected. The only reason was because the velocity factor for the insulated wire was less than that of non-insulated wire. 73, Don W3FPR

> On 12/29/2016 6:33 PM, Walter Underwood wrote: > This is quite likely overly pedantic, but “velocity factor” is a characteristic of transmission lines. Interestingly, it is independent of frequency (up to the limit of the dielectric). It depends on the geometry of the line and the dielectric material. > > Antennas don’t have a velocity factor. The shortened elements are caused by capacitive loading against (RF) ground. There is a percentage of the free-space electrical length due to capacitive loading, but it is not a velocity factor. I don’t think this has a snappy shorthand other than “electrical lengthening due to capacitive loading”. > > For example, dipoles with capacity caps on the ends, like the N6BT designs, don’t change the velocity of propagation along the elements. They use massive capacity loading on the ends (the high-voltage part of the dipole) to shorten the elements. > > http://www.force12inc.com/products/sigma-20-hd-20-meter-heavy-duty-vertical-dipole.html <http://www.force12inc.com/products/sigma-20-hd-20-meter-heavy-duty-vertical-dipole.html> > > OK, overly pedantic mode off, plus I’ve nearly hit the limit of what I remember from my fields and waves class decades ago. I was pretty happy to get a B- in that class. > > wunder > K6WRU > Walter Underwood > CM87wj > http://observer.wunderwood.org/ (my blog) >
>
> Sent from my iPad
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Jim Brown-10

Re: Small QRP antenna

Sorry Jim, you're mistaken on many counts.

An antenna does not have VF, but the wire we use for it does. VF of an
infinitely thin bare wire in free space is 1. Surrounding it with a
dielectric (insulation) makes it longer electrically, usually by a few
percent. So does making the wire fatter. These differences are usually
described using VF.

Placing a conductor close to earth (within a few inches) causes it to
couple to the earth, which also makes it electrically longer. We could
also describe this using VF.

VF is NOT constant with frequency, whether in a transmission line or in
wire. VF of all transmission lines starts out quite small at audio
frequencies, rises quickly through the audio spectrum, eventually
reaching a near constant value at mid-VHF. It is this near-constant
value that is computed by the simple equation that doesn't include
frequency. For most lines, VF has reached about 98% of its final value
at 2 MHz.

Likewise, Zo is not constant with frequency, nor is it a pure
resistance. It starts out quite high at low audio frequencies and is
dominated by capacitance. Zo falls rapidly in the audio spectrum, and is
quite close to its final value at 2 MHz, but is still capacitive,
typically 1-2 ohms.

This can be clearly seen in N6BV's TLW software that comes on the CD
with the ARRL Antenna Book. Choose your favorite coax, set the frequency
to 2 MHz, make the line 300-400 ft long, and terminate it in a pure
resistance. TLW will tell you Zo. Now select Volt/Current next to the
Graph button (lower right corner) and you'll see that there are
standing waves on the line (the graphs are not a straight line). Now set
the Load to the R and X values for Zo and hit Graph again. Now the V and
I lines are nearly straight, indicating quite low SWR. They would lay
exactly on top of each other, but the R and X values for Zo are rounded
off.

The fact that Zo, VF, and attenuation vary with frequency is clearly
predicted by the full transmission line equations. There's a brief
discussion of this in
http://k9yc.com/Coax-Stubs.pdf
which also shows how VF and attenuation can be computed and plotted vs
frequency by making two measurements of a sample with a vector analyzer
like the AIM, SARK, and VNWA analyzers. The two measurements are then
exported to AC6LA's excellent freeware Excel spreadsheet called ZPlots.

There's a longer discussion of this, specifically written for audio
people, but obviously important for radio, in
http://k9yc.com/TransLines-LowFreq.pdf

73, Jim K9YC

On Fri,12/30/2016 11:12 AM, Jim Allen wrote:
> but “velocity factor” is a characteristic of transmission lines. Interestingly, it is independent of frequency (up to the limit of the dielectric). It depends on the geometry of the line and the dielectric material. > > Antennas don’t have a velocity factor. The shortened elements are caused by capacitive loading against (RF) ground. There is a percentage of the free-space electrical length due to capacitive loading, but it is not a velocity factor.

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Jim Allen

Re: Small QRP antenna

In reply to this post by Rick Dwight

Sorry, Jim, you are misquoting the wrong guy. Tweren't me, McGee.

73 Jim Allen W6OGC

> Message: 24
> Date: Fri, 30 Dec 2016 13:25:28 -0800
> From: Jim Brown <[hidden email]>
> To: [hidden email]
> Subject: Re: [Elecraft] Small QRP antenna
> Message-ID:
> <[hidden email]>
> Content-Type: text/plain; charset=utf-8; format=flowed
>
> Sorry Jim, you're mistaken on many counts.
>
> An antenna does not have VF, but the wire we use for it does. VF of an
> infinitely thin bare wire in free space is 1. Surrounding it with a
> dielectric (insulation) makes it longer electrically, usually by a few
> percent. So does making the wire fatter. These differences are usually
> described using VF.
>
> Placing a conductor close to earth (within a few inches) causes it to
> couple to the earth, which also makes it electrically longer. We could
> also describe this using VF.
>
> VF is NOT constant with frequency, whether in a transmission line or in
> wire. VF of all transmission lines starts out quite small at audio
> frequencies, rises quickly through the audio spectrum, eventually
> reaching a near constant value at mid-VHF. It is this near-constant
> value that is computed by the simple equation that doesn't include
> frequency. For most lines, VF has reached about 98% of its final value
> at 2 MHz.
>
> Likewise, Zo is not constant with frequency, nor is it a pure
> resistance. It starts out quite high at low audio frequencies and is
> dominated by capacitance. Zo falls rapidly in the audio spectrum, and is
> quite close to its final value at 2 MHz, but is still capacitive,
> typically 1-2 ohms.
>
> This can be clearly seen in N6BV's TLW software that comes on the CD
> with the ARRL Antenna Book. Choose your favorite coax, set the frequency
> to 2 MHz, make the line 300-400 ft long, and terminate it in a pure
> resistance. TLW will tell you Zo. Now select Volt/Current next to the
> Graph button (lower right corner) and you'll see that there are
> standing waves on the line (the graphs are not a straight line). Now set
> the Load to the R and X values for Zo and hit Graph again. Now the V and
> I lines are nearly straight, indicating quite low SWR. They would lay
> exactly on top of each other, but the R and X values for Zo are rounded
> off.
>
> The fact that Zo, VF, and attenuation vary with frequency is clearly
> predicted by the full transmission line equations. There's a brief
> discussion of this in
> http://k9yc.com/Coax-Stubs.pdf
> which also shows how VF and attenuation can be computed and plotted vs
> frequency by making two measurements of a sample with a vector analyzer
> like the AIM, SARK, and VNWA analyzers. The two measurements are then
> exported to AC6LA's excellent freeware Excel spreadsheet called ZPlots.
>
> There's a longer discussion of this, specifically written for audio
> people, but obviously important for radio, in
> http://k9yc.com/TransLines-LowFreq.pdf
>
> 73, Jim K9YC
>
>> On Fri,12/30/2016 11:12 AM, Jim Allen wrote:
>> but ?velocity factor? is a characteristic of transmission lines. Interestingly, it is independent of frequency (up to the limit of the dielectric). It depends on the geometry of the line and the dielectric material. > > Antennas don?t have a velocity factor. The shortened elements are caused by capacitive loading against (RF) ground. There is a percentage of the free-space electrical length due to capacitive loading, but it is not a velocity factor.
>
> *****************************************