33.25ft vertical with KXAT1 - Why did it work?

[Martin Gillen]

Hi.

I was trying to build a field portable quick to
install
antenna and I really liked what I read about an end
fed
halfwave having a very low angle of radiation and only

requiring a very short counterpoise however
I remembered that the KXAT1 manual stated not to use
lengths near a halfwave as they would be outside the
matching range.

So... I decided that I would cut an antenna to 33ft
(20m halfwave), and then I would cut it back a foot at
a time until the KXAT1 found a match.

I started at 33.25ft vertical but I did leave a pair
of
18.5ft counterpoise wires running at about 2 feet
above the ground connected to the ground terminal.

I tuned up and ... 1.2:1 match on 20m!

I then proceeded to work KF6GC with 2W (almost 4000km)
in the middle of a K Index = 8 solar storm!  So I know
the antenna worked well.

I modelled it in EZNEC and the impedance at the base
of that antenna should have been 970 + j 1700 ohms,
so I am surprised that the KXAT1 was able to match it.

Any comments?  Was it because I left the counterpoises
connected that it worked so well?  If I remove the
counterpoises in the NEC model the impedance increases
and becomes inductive to 1456 - j 1819 ohms.

Comments?

PS.  I am only learning in this stuff so forgive any
overly simplistic analysis!!

Thanks,
Martin.
VA3SIE.


               
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[n3drk]

No reason for any forgiveness. We are all learners. I am awaiting an answer
to this good post also.
john-n3drk


----- Original Message -----
From: "Martin Gillen" <[hidden email]>
To: <[hidden email]>
Sent: Monday, September 12, 2005 12:27 PM
Subject: [Elecraft] 33.25ft vertical with KXAT1 - Why did it work?

> Hi.
>
> I was trying to build a field portable quick to
> install
> antenna and I really liked what I read about an end
> fed
> halfwave having a very low angle of radiation and only
>
> requiring a very short counterpoise however
> I remembered that the KXAT1 manual stated not to use
> lengths near a halfwave as they would be outside the
> matching range.
>
> So... I decided that I would cut an antenna to 33ft
> (20m halfwave), and then I would cut it back a foot at
> a time until the KXAT1 found a match.
>
> I started at 33.25ft vertical but I did leave a pair
> of
> 18.5ft counterpoise wires running at about 2 feet
> above the ground connected to the ground terminal.
>
> I tuned up and ... 1.2:1 match on 20m!
>
> I then proceeded to work KF6GC with 2W (almost 4000km)
> in the middle of a K Index = 8 solar storm!  So I know
> the antenna worked well.
>
> I modelled it in EZNEC and the impedance at the base
> of that antenna should have been 970 + j 1700 ohms,
> so I am surprised that the KXAT1 was able to match it.
>
> Any comments?  Was it because I left the counterpoises
> connected that it worked so well?  If I remove the
> counterpoises in the NEC model the impedance increases
> and becomes inductive to 1456 - j 1819 ohms.
>
> Comments?
>
> PS.  I am only learning in this stuff so forgive any
> overly simplistic analysis!!
>
> Thanks,
> Martin.
> VA3SIE.
>
>
>
> ___________________________________________________________
> To help you stay safe and secure online, we've developed the all new
> Yahoo! Security Centre. http://uk.security.yahoo.com
> _______________________________________________
> 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
>

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[Stephen W. Kercel]

Martin:

Antennas work by witchcraft and magic.  NEC is about as good a modeling
program as you will find, but all antenna calculations are only a gross
indication of expected performance. Actually, the problem is that the near
field has extreme sensitivity to boundary conditions; these are impossible
to model realistically, but they do affect performance measures such as
feedpoint impedance. All antenna analyses are overly simplistic compared to
the real antenna that they represent. Thus, comparing NEC studies to
observed results often gives the appearance of witchcraft and magic.

Three things might account for your result. First, the idealized
description of the antenna in NEC tends to depart from reality when you are
trying to look at impedance for a feedpoint at the voltage loop. (Technical
detail: Near the current loop, where dipoles are usually fed, the voltage
along the antenna varies slowly with displacement. Thus, NEC's assumption
that the voltage is uniform along a segment is approximately correct, and
estimates of impedance are pretty close. Near the voltage loop, where
you're feeding your antenna, the voltage along the antenna varies quickly
with displacement. Thus, NEC's assumption that the voltage is uniform along
the segment is far from correct, and estimates of impedance can be way
off.  ) In my experience, the NEC estimate of impedance for voltage loop
feedpoints is often overstated by a factor of 2 to 3. Second, your
counterpoise is very near the ground, and the description of ground in
EZNEC is (unavoidably) very crude. Third, you did not say how long your
feedline is. A lossy feedline will have a non-trivially lower SWR at
transmitter end than at the antenna end.

If you're getting 2000 km/watt in a K=8 storm, you've got a good
antenna/feedline system.

Congratulations and 73,

Steve
AA4AK


At 05:27 PM 9/12/2005 +0100, Martin Gillen wrote:

>Hi.
>
>I was trying to build a field portable quick to
>install
>antenna and I really liked what I read about an end
>fed
>halfwave having a very low angle of radiation and only
>
>requiring a very short counterpoise however
>I remembered that the KXAT1 manual stated not to use
>lengths near a halfwave as they would be outside the
>matching range.
>
>So... I decided that I would cut an antenna to 33ft
>(20m halfwave), and then I would cut it back a foot at
>a time until the KXAT1 found a match.
>
>I started at 33.25ft vertical but I did leave a pair
>of
>18.5ft counterpoise wires running at about 2 feet
>above the ground connected to the ground terminal.
>
>I tuned up and ... 1.2:1 match on 20m!
>
>I then proceeded to work KF6GC with 2W (almost 4000km)
>in the middle of a K Index = 8 solar storm!  So I know
>the antenna worked well.
>
>I modelled it in EZNEC and the impedance at the base
>of that antenna should have been 970 + j 1700 ohms,
>so I am surprised that the KXAT1 was able to match it.
>
>Any comments?  Was it because I left the counterpoises
>connected that it worked so well?  If I remove the
>counterpoises in the NEC model the impedance increases
>and becomes inductive to 1456 - j 1819 ohms.
>
>Comments?
>
>PS.  I am only learning in this stuff so forgive any
>overly simplistic analysis!!
>
>Thanks,
>Martin.
>VA3SIE.
>
>
>
>___________________________________________________________
>To help you stay safe and secure online, we've developed the all new
>Yahoo! Security Centre. http://uk.security.yahoo.com
>_______________________________________________
>Elecraft mailing list
>Post to: [hidden email]
>You must be a subscriber to post to the list.
>Subscriber Info (Addr. Change, sub, unsub etc.):
>  http://mailman.qth.net/mailman/listinfo/elecraft
>
>Help: http://mailman.qth.net/subscribers.htm
>Elecraft web page: http://www.elecraft.com

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[Ron D'Eau Claire-2]

Martin VA3SIE wrote:

I really liked what I read about an end fed halfwave having a very low angle
of radiation and only requiring a very short counterpoise however I
remembered that the KXAT1 manual stated not to use lengths near a halfwave
as they would be outside the matching range.

So... I decided that I would cut an antenna to 33ft (20m halfwave), and then
I would cut it back a foot at a time until the KXAT1 found a match.

I started at 33.25ft vertical but I did leave a pair of 18.5ft counterpoise
wires running at about 2 feet above the ground connected to the ground
terminal.

I tuned up and ... 1.2:1 match on 20m!

I then proceeded to work KF6GC with 2W (almost 4000km) in the middle of a K
Index = 8 solar storm!  So I know the antenna worked well.

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

The counterpoise is part of the antenna. (Think an off-center-fed wire
without feeder.) So you started out with a nearly 52 foot long radiator as
far as the KXAT1 was concerned.

Longer is usually better. First, you do have lower ground losses. And, yes,
when the radiator is exactly 1/2 wave long the impedance will be very high,
beyond what the KXAT1 can handle, so approaching it as close as possible is
a good plan except for one thing. That issue is that the rig will end up at
a voltage "loop" meaning that touching the rig will detune the system badly.
The counterpoise exhibits a fairly low impedance to keep the case of the rig
near RF 'ground'.

You mention a low angle of radiation. The pattern is entirely dependent upon
the antenna's position with respect to the earth or other conductors. The
manner of feeding the antenna has nothing to do with the pattern. An end fed
half wave (dipole) has a pattern identical to a center-fed half wave. In
free space a half wave radiator has maximum radiation at right angles to the
wire. Stand it on end over the earth and I'll show very low angles of
radiation out toward the horizon, BUT very little radiation down below 20
degrees in most cases. That's because the earth is not a good conductor.
Most of the RF at low angles is used up in earth losses. Those losses occur
at several wavelengths from the antenna, so even a hundred "radials" won't
stop that action. Put the antenna horizontally over the earth and its height
above the earth has a dramatic effect on the radiation pattern. Keep it low
- about 0.2 wavelengths - and it forms a two-element "beam" using the earth
as a reflector producing a huge lobe straight up. That's what's often called
an NVIS (near vertical incidence system) nowadays. Raise the horizontal up
to about 1/2 wave above the earth and that huge vertical lobe splits out and
drops closer to the horizon, producing great low-angle radiation for DX and,
because the radiation is horizontally polarized, the ground losses are less
than with a vertical. Such a horizontal antenna can show BIG gains - about 6
dB. That means a K2/100 feeding such an antenna can put out a signal
equivalent to a 400 watt rig feeding a unity gain antenna like a ground
plane. The problem is that most of us can't get a horizontal up 1/2 wave:
130 feet on 80 or even 66 feet on 40. So we live with a compromise.
Sometimes a vertical is better for DX than a low horizontal wire.

Were radials come into play is when the antenna is too short to be
self-resonant. The shortest self-resonant antenna is a dipole (1/2 wave long
radiator). When a radiator is shorter than 1/2 wave (a so-called "Marconi"
antenna because that's what he used), the ground system has to take over the
duty of providing a current sink that would normally have been provided by
the missing length. The lower the impedance of this ground system, the more
efficient the antenna. That's why people using radiators shorter than 1/2
wavelength, no matter vertical or horizontal, try to do everything they can
to provide the best possible "ground". AM broadcast stations, for example,
have for years used 120 radials 0.2 wavelengths long for this purpose.

It's also why ships have such great success with short antennas. On the old
600 meter "marine" band (500 kHz), a half wave antenna would be almost 1,000
feet long! No ship I ever saw could provide room for a wire even half that
long; usually  much, much less. Still they "got out" very well, and on the
short waves their signals were downright astounding compared to shore
stations. That's because salt water is just about the best natural ground
plane one can expect to find on the planet, and a ship at sea is sitting in
the middle of one that extends for thousands of wavelengths in all
directions!

Ron AC7AC

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[Leigh L. Klotz Jr WA5ZNU]

Martinm
You can use the ATU menus to find out what value L and C and which
network configuration NT1 or NT2 was in use by the KX1's ATU and you can
see what the L network looks like in each case in the manual.

See http://del.icio.us/tag/EFHWA for more links on end-fed half-wave
antennas, from people who have field experience.

Leigh / WA5ZNU

On Mon, 12 Sep 2005 9:42 pm, Martin Gillen wrote:

> Hi.
>
> I started at 33.25ft vertical but I did leave a pair of 18.5ft
> counterpoise wires running at about 2 feet above the ground connected
> to the ground terminal.
>
> I tuned up and ... 1.2:1 match on 20m!
>
> I then proceeded to work KF6GC with 2W (almost 4000km)
> in the middle of a K Index = 8 solar storm!  So I know
> the antenna worked well.
>
> I modelled it in EZNEC and the impedance at the base
> of that antenna should have been 970 + j 1700 ohms,
> so I am surprised that the KXAT1 was able to match it.
>
> Any comments?  Was it because I left the counterpoises
> connected that it worked so well?  If I remove the
> counterpoises in the NEC model the impedance increases
> and becomes inductive to 1456 - j 1819 ohms.
>
> Comments?
>
> PS.  I am only learning in this stuff so forgive any
> overly simplistic analysis!!
>
> Thanks,
> Martin.
> VA3SIE.
>
>
>
> ___________________________________________________________
> To help you stay safe and secure online, we've developed the all new
> Yahoo! Security Centre. http://uk.security.yahoo.com
> _______________________________________________
> Elecraft mailing list
> Post to: [hidden email]
> You must be a subscriber to post to the list.
> Subscriber Info (Addr. Change, sub, unsub etc.):
>  http://mailman.qth.net/mailman/listinfo/elecraft
>
> Help: http://mailman.qth.net/subscribers.htm
> Elecraft web page: http://www.elecraft.com

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