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Elecraft: RFI Problems

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Martin.Evans

Elecraft: RFI Problems

Sorry this is "off topic" but there is so much experience here. I've been
licensed for only 2 years, my only rig is K1 so I've been QRP CW only (and
enjoyed it immensely). I've recently started using a PA to get up to 100W
on 3.5 to 21MHz, but RFI problems have started to appear. Here are the
symptoms:

50Hz hum on key down on hi fi
Similar problem on mains MW radio.
Freezing of PC modem on key down.

These are all in different rooms of the house and all at least 15 feet away
from the rig.

The rig is on the ground floor right next to the outside wall where the
coax feeder runs out at ground level. Feeder is about 70 feet long and all
at ground level until it rises to a multi dipole at 30 feet up, cut for
80/40/20m. Feed point is at least 50 feet from house. The feeder shield is
connected directly to the PA ground point and therefore to the mains ground
wire. I have no Balun and no RF ground so I guess I've broken all the
rules.

I suspect RF is on the mains 240V house wiring and can see it on my scope
(about 5mV P-P superimposed on the mains). I have a number of things to try
but not sure in what order of priority, maybe someone could help, I want to
tackle the source of the RFI first, rather than the affected domestic
equipment.

Here's the list:
Make a coax choke balun in the feeder to reduce out of balance currents
(quick and cheap)
Fit a commercial balun (heavy and pricey)
Construct an RF ground in the garden and connect to feeder sheath at entry
into the house. (time consuming)
Fit a ferrite ring choke to the PA mains lead
Build an inductively coupled ATU to avoid connection of feeder sheath to
the mains ground.
Fit ferrite ring chokes to the affected equipment
Fit a proprietary RFI filter to the telephone line (to cure the modem
problem)

Or maybe I should just go back to QRP hi.

Any advice would be greatly appreciated as I don't want to run the risk of
causing RFI to the neighbours.

Why not take a look at our Web site?
http://www.simoncarves.com

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Stewart Baker

Re: Elecraft: RFI Problems

On Fri, 28 Jan 2005 09:59:35 +0000, [hidden email] wrote:

>
>
> Sorry this is "off topic" but there is so much experience here. I've been
> licensed for only 2 years, my only rig is K1 so I've been QRP CW only (and
> enjoyed it immensely). I've recently started using a PA to get up to 100W
> on 3.5 to 21MHz, but RFI problems have started to appear. Here are the
> symptoms:
>
> 50Hz hum on key down on hi fi
> Similar problem on mains MW radio.
> Freezing of PC modem on key down.
>
> These are all in different rooms of the house and all at least 15 feet away
> from the rig.
>
> The rig is on the ground floor right next to the outside wall where the
> coax feeder runs out at ground level. Feeder is about 70 feet long and all
> at ground level until it rises to a multi dipole at 30 feet up, cut for
> 80/40/20m. Feed point is at least 50 feet from house. The feeder shield is
> connected directly to the PA ground point and therefore to the mains ground
> wire. I have no Balun and no RF ground so I guess I've broken all the
> rules.
>
> I suspect RF is on the mains 240V house wiring and can see it on my scope
> (about 5mV P-P superimposed on the mains). I have a number of things to try
> but not sure in what order of priority, maybe someone could help, I want to
> tackle the source of the RFI first, rather than the affected domestic
> equipment.
>
> Here's the list:
> Make a coax choke balun in the feeder to reduce out of balance currents
> (quick and cheap)
> Fit a commercial balun (heavy and pricey)
> Construct an RF ground in the garden and connect to feeder sheath at entry
> into the house. (time consuming)
> Fit a ferrite ring choke to the PA mains lead
> Build an inductively coupled ATU to avoid connection of feeder sheath to
> the mains ground.
> Fit ferrite ring chokes to the affected equipment
> Fit a proprietary RFI filter to the telephone line (to cure the modem
> problem)
>
> Or maybe I should just go back to QRP hi.
>
> Any advice would be greatly appreciated as I don't want to run the risk of
> causing RFI to the neighbours.
>
>
> Why not take a look at our Web site?
> http://www.simoncarves.com
>
> *************************************************************************
> The information in this email and any attachments may contain
> privileged and/or confidential information intended solely for
> the use of the individual or entity to whom it is addressed.
> If the reader of this email is not the intended addressee, or the
> employee or agent responsible to deliver it to the addressee,
> you are hereby notified that any dissemination, distribution or
> copying of this communication is strictly prohibited.
> If you have received this communication in error, please notify
> me by telephone or email and delete all copies immediately.
> *************************************************************************
> _______________________________________________
> Elecraft mailing list
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> You must be a subscriber to post to the list.
> Subscriber Info (Addr. Change, sub, unsub etc.):
> http://mailman.qth.net/mailman/listinfo/elecraft
>
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> Elecraft web page: http://www.elecraft.com

Things that I would try first:-

a) Fit a balun (at the center of the dipole)
or
b) Make the antenna completely balanced by using open wire feeder, and a
balanced ATU. You will notice a reduction in received noise as well as RFI
reduction.

Good luck

73
Stewart G3RXQ

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Don Wilhelm-3

Re: Elecraft: RFI Problems

In reply to this post by Martin.Evans

The short answer to your problem is - keep the RF inside the coax until it
gets to the antenna.
With an unbalanced feedline and no balun, you likely have considerable RF on
the outside of the coax.
Your list of possible cures is a good one, but add one more at the top - run
the feedline away from the antenna at right angles for as far as possible -
that places the feedline in a position where the fields should cancel and
prevents the feedline from picking up radiation from one side of the
antenna.
Then start at the top of your list and try each item one at a time until you
have found what is effective.

If your antenna is located close to the equipment you are having difficulty
with, as might be the case with a low antenna or an attic antenna, then QRP
may be you only solution. Those devices are supposed to be unaffected by an
RF field, but we all know what is supposed to be and what really is are two
different things.

Your neighbors should be further away from the antenna than your own
equipment is located, so your local problem should be greater than theirs.

73,
Don W3FPR

----- Original Message -----

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Vic K2VCO

Re: Elecraft: RFI Problems

In reply to this post by Martin.Evans

[hidden email] wrote:

> 50Hz hum on key down on hi fi
> Similar problem on mains MW radio.
> Freezing of PC modem on key down.

My suggestions, in order:

1) Try wrapping the speaker leads of your hi fi around ferrite rods or toroids.
If this doesn't clear it up, try wrapping the mains cord from the hi fi around
a ferrite as well. Try to get as many turns as possble. The clamp-on beads are
not effective at HF.

2) Treat the radio in a similar fashion.

3) Use an inline telephone filter (such as the k-com or similar) on your modem
phone line. Also wrap the power wire to the modem around a ferrite.

4) Add a balun of some kind to your dipole.

There's a lot more that you can do, but this basic stuff should help.

--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco

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Stephen W. Kercel

Re: Elecraft: RFI Problems

In reply to this post by Martin.Evans

Martin:

I have a few thoughts on your EMI problem.

First, keep in mind that an antenna produces three different kinds of
fields, the induction field, the near field and the far field. The
induction field vanishes within inches of the antenna. The near field, the
cause of many emi problems, has non-trivial strength out to about 1/6 of a
wavelength. The far field is the one that "gets out," and behaves like a
nice predictable travelling wave. The near field turns out to be extremely
sensitive to boundary conditions, and simple acts like opening or closing a
door can change the boundary enough to change the pattern of the near
field. This is one reason for the seemingly "magical" properties of emi.

My reason for making this disclaimer is to make you appreciate the fact
that any of the very sound advice you get on the reflector may or may not
help you to overcome your particular emi problem. I would encourage you not
to give it up; your problem (or set of seemingly unrelated problems) can be
solved.

Second, you have not mentioned what bands give you the emi problem. You
should check for all the symptoms on all the bands. There is a good
possibility that part of your house wiring is perversely resonant in one of
the ham bands, and is acting like an antenna that captures and reradiates
your near field. Of course, if the resonance occurs on 80 meters, you might
also be resonant on all the harmonically related ham bands. Anyway, it is
worth checking which bands cause which bad effects.

Third, having no RF ground is a big nono. The problem is that the RF that
is on the chassis of your rig has to go somewhere. If you do not provide it
a low impedance path to ground (where RF is converted to heat) it will find
its own path, and the one that it finds will not make you happy. The power
system ground has very high inductive reactance (A straight wire has an
inductance of 10 nanohenries per inch. A 10 foot length of wire has an
inductive reactance of 26 ohms at 3.5 MHz in the absence of mitigating
capacitive effects. ) In other words, from your rig to the power system
ground rod the overall impedance is probably on the order of hundreds of
ohms, and virtually none of your RF energy is finding its way to the ground
rod. If you're lucky (in your case you're not) the energy will be turned
into heat in the losses of the wiring. Otherwise, it gets coupled into your
other equipment.

Since you are physically at ground level, I would strongly recommend that
you install an 8 foot copperweld ground rod as close as possible to the
rig, and feed through the shortest possible number 6 solid wire. (Solid has
lower inductance than stranded or braid.) This is not an expensive
solution, but it may solve your problem. Going from the shield of your
cable direct to the ground rod may solve your problem and it may not. The
emi problem may not be caused by radiation emanating from the outside of
your shield. To avoid the creation of ground loops, and new emi problems,
it is considered best practice to use a single point RF ground. Connect the
rf grounds of your auxiliary equipment to your antenna tuner ground if you
have one or to your transmitter ground otherwise. Make sure the ground
connections are done in a star configuration (no loops in the ground wire),
with short direct connections of solid wire (braid has higher inductance;
avoid it) with your rig or tuner ground being the center of the star. To
that single point at the center of the star connect the lead to the RF
ground. This is a compromise configuration that tries to balance many
conflicting tradeoffs, but it is the one that typically results in the
fewest emi problems.

The 8 foot copperweld ground rod is not really that good an electrode at
HF, but it is better than nothing and vastly better than what you have now.
In fact, there is no good ground electrode at HF. All of them have
non-trivial inductive reactance. The least electrically bad ground
electrode is a long solid copper wire (I prefer number 6; it has good
physical strength and is not too hard to handle) at least a half wavelength
at your lowest frequency and buried just deep enough that your lawn mower
does not hit it, but preferably less than an inch deep. Every 8 feet or so
along the wire you should connect the wire to a 12 inch ground rod. The
wire does not need to be in a straight line. It can meander all over the
yard, turning corners as necessary, but no corner should be tighter than 90
degrees. Obviously this is not practical to install during the winter.
Nevertheless, when you can install such a ground, it is worth the trouble
doing.

BTW, a cheap trick that often works in place of an RF ground is to use an
MFJ (or equivalent) artificial ground to a length of wire running along the
floor as an RF counterpoise. The idea is that the reactive elements in the
box tune out the reactance in the counterpoise and create the effect of a
low impedance RF ground. You can find artificial grounds pretty cheap on eBay.

You note that "these are all in different rooms of the house and all at
least 15 feet away from the rig." Since the emi is by your own observation
being coupled into the house wiring, distance from the rig is irrelevant.
If you're lucky, the losses in the power company's equipment where your
house connects to the system is attenuating the emi, and probably none of
the energy is going out on their transmission line and into other peoples
houses.

Personally, I would be looking for a systemic cause and cure rather than
cleaning up individual symptoms. You might clean up everything you have,
but the next new piece of gear that you bring into the house may cause you
a new set of problems.

Some comments on your proposed fixes:

"Make a coax choke balun in the feeder to reduce out of balance currents
(quick and cheap)
Fit a commercial balun (heavy and pricey)"

Unless you're feeding the dipole off center, or your transmission line is
severely off perpendicular to your antenna, or you have some very gross
asymmetry in the boundary conditions (like your house having a metal roof),
your configuration does not look like it is especially vulnerable to the
problem. Unless there is something causing you have RF on the outside of
your coax sheath, the baluns probably will not buy you anything.

"Construct an RF ground in the garden and connect to feeder sheath at entry
into the house. (time consuming)"

Time consuming, but probably worth it. However, as I already noted, you do
not appear to have a compelling reason to believe that you have a line
radiation problem. I expect that you'd be better off using the single point
connection scheme I described above.

"Fit a ferrite ring choke to the PA mains lead"

This is a cheap trick, but it might work. There are a couple of gotchas.
First, in the absence of an RF ground connection, you have no low impedance
path that takes the emi energy to a place where it is converted to heat.
You're simply redirecting a bit of bad energy, and the practical result may
be to trade one emi problem for another. However, you may find that you
need both the ferrite and the RF ground working in concert to solve the
problem. If by a ferrite ring you mean those little rectangular jobbies
that you get from MFJ or Radio Shack, be aware that they do not provide
much attenuation, about 3-5 dB per turn at HF. You need to go 7 times
through the core to get significant attenuation, maybe 7 turns through one
core, or a stack of 7 cores on a straight wire, or some other combination.

"Build an inductively coupled ATU to avoid connection of feeder sheath to
the mains ground."

You'd only avoid a DC connection. If you do not provide an RF ground, and
if you do have feedline radiation, the system will probably find some other
coupling mechanism to couple energy from the feedline to your house wiring.

"Fit ferrite ring chokes to the affected equipment"

Again, it might work. However, those rings are about four dollars each, and
you might need quite a few on each piece of equipment. The price could grow
in a hurry. You're probably much better off trying to find the mechanism
that's coupling RF into your power system and breaking that up.

"Fit a proprietary RFI filter to the telephone line (to cure the modem
problem)"

This will only work if the problem is that RF on the phone line is being
coupled into the modem and being demodulated by the modem to create the
local emi effect. You already know that you've got emi on the power line,
if your emi is coming in through the power lead, the telephone filter would
not do you any good; you'd be locking the wrong door. You might want to put
the scope on the phone wires and see if you have RF there. Those telephone
filters are about 11 dollars each. You do not want to make a big investment
in them unless you have reason to believe they will actually solve your
problem.

Good luck with your problem, and please keep us posted on your progress.

73,

Steve
AA4AK

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Vic K2VCO

Re: Elecraft: RFI Problems

Stephen W. Kercel wrote:

> Third, having no RF ground is a big nono. The problem is that the RF
> that is on the chassis of your rig has to go somewhere.

I'm sorry, I have to respectfully disagree. If you have a balanced antenna
system, or even an unbalanced system with RF properly choked off from the
outside of your coax, there will be no RF on the chassis that 'as to go
somewhere'.

In addition, it is very difficult to provide an RF ground at HF. Even the
8-foot wire you mention will have significant impedance. And a ground rod is of
absolutely NO use for RF grounding, although correctly applied ground rods may
be useful for lightning protection.

--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco
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Stephen W. Kercel

Re: Elecraft: RFI Problems

Vic:

Because there is so much going on in EMI, and so much of that, being
extremely sensitive to boundary conditions, is unique to a specific
situation, there will always be room for respectful disagreement as to how
to solve or prevent problems.

If radiation from the exterior surface of the antenna feedline were the
only mechanism that causes emi, then you would be right, choking it off
would solve the emi problem. The energy in the transmission line would be
radiated by the antenna or turned into heat by either the line losses or by
the losses in the materials used to make up the choke. However, emi can be
caused by many other mechanisms, and as one other poster to the reflector
has reported, improving his RF ground substantially mitigated his problem.

I do agree that an effective RF ground is hideously difficult to achieve at
HF. I did not recommend using an 8 foot rod as an RF ground. I merely
observed that it is better than nothing. I admit that an 8 foot ground rod
with a short lead would be a quarter-wavelength, and thus would look like a
perfect open circuit at 10 meters. On the other hand, for typical soil, at
80 meters it would present a complex impedance on the order of 15 + J 25
Ohms. That is uncomfortably high, but far better than nothing.

The "least bad" HF electrode that I reported in my previous post was based
on an Army/Air Force study about 20 years ago. Whether or not it is worth
the considerable trouble needed to install it is a judgment call. I did
install such a system, and if I had it to do again, I definitely would. I
have had no emi problems at my station, despite the fact that my QTH is
bristling with all sorts of new-fangled "smart house" electronics.

73,

Steve
AA4AK
Brunswick ME

At 08:33 AM 1/29/2005 -0800, you wrote:

>Stephen W. Kercel wrote:
>
>>Third, having no RF ground is a big nono. The problem is that the RF that
>>is on the chassis of your rig has to go somewhere.
>
>I'm sorry, I have to respectfully disagree. If you have a balanced
>antenna system, or even an unbalanced system with RF properly choked off
>from the outside of your coax, there will be no RF on the chassis that 'as
>to go somewhere'.
>
>In addition, it is very difficult to provide an RF ground at HF. Even the
>8-foot wire you mention will have significant impedance. And a ground rod
>is of absolutely NO use for RF grounding, although correctly applied
>ground rods may be useful for lightning protection.
>
>--
>73,
>Vic, K2VCO
>Fresno CA
>http://www.qsl.net/k2vco

G3VVT

Re: Elecraft: RFI Problems

In reply to this post by Martin.Evans

In a message dated 29/01/05 16:36:57 GMT Standard Time, [hidden email]
writes:

If you have a balanced antenna
system, or even an unbalanced system with RF properly choked off from the
outside of your coax, there will be no RF on the chassis that 'as to go
somewhere'.

In addition, it is very difficult to provide an RF ground at HF.

---------------------------------------------------------

With my shack in an upstairs spare bedroom it is difficult to provide an RF
ground in any case, the only ground being the power ground. The house is
built on rock so I doubt if a ground rod would go in very deep even if I tried.

Opted like Vic for a balanced antenna with balanced feeder. Using 100W have
not seen any problems unless I try to operate on a band that the antenna is
not resonant on. Using 75 ohm balanced twin as the feeder as the use of 300 ohm
ribbon or higher would be impractical from my shack. The 75 ohm twin seems
quite docile in use and can be installed in close proximity to metallic
objects which is not recommended for 300 ohm or higher impedance twin feeder.

To date the only interference problem have been with telephones whose drop
wires are parallel to my antenna. The phone company fitted RF filters to the
affected telephones at the point of entry into the house and one modern
telephone had to be suppressed internally. Older telephones seem less affected. A
problem that may be coming up is the use of ADSL broadband would probably
prevent normal telephone RF suppressers being used at the point of entry to the
house as this would also stop ADSL from working. A separate RF filter at each
telephone would be the only answer in this case.

As a parallel subject there was an interesting article in the February 2005
Radcom, EMC column advocating the use of ADSL telephone filters as being very
suitable for use as RF filters on telephones for the lower bands and the
performance figures for one type of ADSL filter were quoted. See them available
at computer fairs at about £3 each ($5.70 US).

Bob, G3VVT
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Stephen W. Kercel

Re: Elecraft: RFI Problems

In reply to this post by Stephen W. Kercel

Vic:

Certainly it is the case that the remedy that solves one emi problem might
make another one worse. That is what makes emi so difficult.

73,

Steve
AA4AK
Brunswick ME

At 04:33 PM 1/29/2005 -0800, you wrote:

>Stephen W. Kercel wrote:
>
>>If radiation from the exterior surface of the antenna feedline were the
>>only mechanism that causes emi, then you would be right, choking it off
>>would solve the emi problem. The energy in the transmission line would be
>>radiated by the antenna or turned into heat by either the line losses or
>>by the losses in the materials used to make up the choke. However, emi
>>can be caused by many other mechanisms, and as one other poster to the
>>reflector has reported, improving his RF ground substantially mitigated
>>his problem.
>
>I understand that there are other paths, such as pickup on the
>power/phone/speaker lines directly from the antenna, etc. This is why I
>suggested that the fellow use ferrites on the power leads to his stereo
>and modem. My point about choking off RF on the feedline was just that an
>RF-free feedline is easier to achieve (unless you are using a random wire
>antenna) than an RF ground.
>
>I have to say that the guy who reported an improvement may have gotten an
>improvement -- but not from the fact that his rig had a better ground,
>rather from the change in the RF environment that his ground system
>caused. In other words, it might make it better, but it also could make
>it worse.
>
>--
>73,
>Vic, K2VCO
>Fresno CA
>http://www.qsl.net/k2vco
>

Ron D'Eau Claire-2

RE: Elecraft: RFI Problems

In reply to this post by G3VVT

Bob, G3VVT wrote:

...Opted like Vic for a balanced antenna with balanced feeder. Using 100W
have
not seen any problems unless I try to operate on a band that the antenna is

not resonant on. Using 75 ohm balanced twin as the feeder as the use of 300
ohm
ribbon or higher would be impractical from my shack. The 75 ohm twin seems
quite docile in use and can be installed in close proximity to metallic
objects which is not recommended for 300 ohm or higher impedance twin
feeder.
-------------------------

All of the 75 ohm stuff that I've seen wasn't rated for high voltages
anyway, so it's likely to melt at the voltage loop when hit with high power.
AT the very least, it'll be rather lossy.

The "rule of thumb" for any balanced line is to keep it at least 3 or 4
times the wire spacing from objects. With the narrow-spaced 75 ohm line
that's much easier than using twin lead or open wire lines as you noted. I'm
lucky to have my operating desk right at a window directly below the center
of my doublet, so my 2.5-inch spaced open wire feeders can come right down
to a header mounted in the window and into my balanced ATU. That's about the
only way one can deal with wide-spaced lines effectively <G>.

Even with the balanced line, if you happen to have a voltage loop - high RF
voltage point on the feeder - at the ATU output, you may see the whole rig
tend to "float" at the high RF voltage, producing some undesirable effects:
RF in the shack and RF feedback in nearby equipment in the house. One
approach is to cut the feedline to a length that makes the feed
low-impedance. Adding or subtracting up to a 1/4 wavelength on a problematic
band will do that. Often less will do fine. Another approach is the
time-tested 1/4 wave long wire attached to the rig chassis and insulated at
the far end. The far end will be very hot with RF and must be well
insulated, but it will show a low impedance and low RF voltage at the rig.

If by "resonant" you mean your antenna is 1/2 wave long, then feeding it in
the center will produce a decent match for 75 ohm line and the RF voltage
should be low all along it.

Ron AC7AC

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G3VVT

Re: Elecraft: RFI Problems

In reply to this post by Martin.Evans

In a message dated 30/01/05 01:32:53 GMT Standard Time,
[hidden email] writes:

All of the 75 ohm stuff that I've seen wasn't rated for high voltages
anyway, so it's likely to melt at the voltage loop when hit with high power.
AT the very least, it'll be rather lossy.

---------------------------------------

There is more robust USA sourced 1kW rated 75 ohm twin available in the
market though sadly not as easy to obtain now as in the past. Believe red/black
power cable makes an effective 75 ohm twin feeder and this is available for
quite high currents though not with high voltages. Am using commercial 75 ohm
twin feeder cable rated at 300W at present which is very adequate for the 100W
maximum I use. In any case we have in the UK a maximum license power of 400W
pep which restricts our exposure to high power effects. Guess would be
rather different in the USA with your maximum of 2kW pep.

QRO can have it's downside!

Not sure that the statement that it would be rather lossy is correct as I
have been led to believe that twin feeder is less lossy than coax and
particularly the 50 ohm version. Even the 75 ohm hard-line coax as used by cable TV
companies was always better than the 50 ohm coax in the loss figures for a
given diameter cable.

I do use a resonant 1/2 wave trap dipole on 40/80m only at present with the
feeder cut to the length recommended by the antenna designer of 70 ft (or
multiples of this). Working towards adding a two extra pairs of traps for 80/20
to give added bands of 160m and 20m. With always using resonant antennas the
problems that could arise on the twin feeder are minimised and hopefully any
interference problems as a result. So far telephones seem the only problem
area.

The wider spaced twin feeder would be impossible in my case as the operating
position is on the opposite side of the room from the window and built into
a fitted wardrobe. At a previous house the shack was just that, a wood hut
under the centre of the dipole and wide spaced twin feeder came direct to a
balanced ATU.

As always it is a matter of "cutting the cloth to suit your circumstances".
What will be suitable for one situation may be hostile in another.

Bob, G3VVT
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Ron D'Eau Claire-2

RE: Elecraft: RFI Problems

Bob, G3VVT wrote:

Not sure that the statement that it would be rather lossy is correct as I
have been led to believe that twin feeder is less lossy than coax and
particularly the 50 ohm version. Even the 75 ohm hard-line coax as used by
cable TV companies was always better than the 50 ohm coax in the loss
figures for a given diameter cable.

--------------------------------------------

Quite true about the relative losses, Bob. We've kicked this subject around
here in the past. It's been pointed out that, while it is true that there
are dielectric losses in feedlines, feedlines operating at high SWR's are
more prone to resistive losses in the conductors than dielectric losses.

Several people have pointed to the fact that the losses are directly
proportional to SWR as shown in all the transmission line loss charts,
suggesting that what the loss charts are showing are the increased ohmic
losses occurring at the current maxima points (current loops) along the
transmission line as the SWR increases.

Voltages and currents do depend upon power, but low power does not guarantee
low currents or voltages on a feedline operating with a high SWR. I melted a
nylon banana jack running 15 watts! The jack was in a grounded panel
connecting my ATU to a voltage-fed antenna. On the other hand, very high
currents can occur at the current loop in a feedline operating at high SWR
even at very low power levels.

That's why "flat" feedlines having little or no standing waves are less
lossy. The current and voltage are directly proportional to the power
applied and do not swing through large extremes along the length of the
feedline.

Feedlines with larger conductors fare better in general. As you pointed
out, "hard line" and other heavy duty transmission lines have excellent
insulation. They also tend to have larger conductors with more surface area
to reduce the resistance to the RF flowing along the surface.

I count myself lucky to be able to use open wire line with #12 conductors.
We all have to deal with compromises. But that's half the challenge of the
hobby - finding the compromise that produces the best results! In my case
it's no high supports nor length enough! (Great feedline, marginal antenna!)

A lot of modern telephones are incredibly RF-sensitive, having amplifiers
and other circuits that have absolutely no RF protection. Your residual
problem may be simply direct pickup by the phone wires of the RF signal
acting as an antenna.

Ron AC7AC

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Don Wilhelm-3

Re: Elecraft: RFI Problems

Ron, Bob and all,

It is quite true that losses increase as the SWR increases - the ohmic
losses are the contributing factor with increased current, but the
dielectric losses will be important as the voltage increases - so both are a
contributing factor. How much contribution is due to the ohmic loss or the
dielectric loss is dependent on the feedline construction and frequency.
With smaller feedlines and lower frequencies, the ohmic losses are likely to
predominate, but as the physical size of the feedline increases, the
dielectric losses may become the greater factor. The crossover point
depends on frequency - and at HF, the ohmic loss usually predominates.

Perhaps I am being overly technical here - and it really may not be
important - if you have losses, you simply generate heat instead of RF -
these subtle points are only appropriate if you want to persure the proper
path to reduce the loss. If you have already made the compromise choices
that are right for your station, there is not much that can be done in the
way of improvement - just understand your choices (decisions).

If you want to adopt a 'rule of thumb', a feedline with a higher
characteristic impedance will have lower loss when comparing feedlines of
similar size. Even though the center conductor of 75 ohm (RG-11) has a
smaller diameter that 50 ohm (RG-8) coax - compare Belden 8237 with #13
center conductor with Belden 8213 with a smaller (#14) center conductor and
you will see that the 75 ohm Belden 8213 has lower loss at higher
frequencies even though the ohmic loss through the #13 center conductor will
be smaller - the higher impedance cable wins in the loss game. Several
other comparisons will reveal similar results.

73,
Don W3FPR

----- Original Message -----
From: "Ron D'Eau Claire" <[hidden email]>

Quite true about the relative losses, Bob. We've kicked this subject around
here in the past. It's been pointed out that, while it is true that there
are dielectric losses in feedlines, feedlines operating at high SWR's are
more prone to resistive losses in the conductors than dielectric losses.

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