[Elecraft] KX3 Field Ant. for 80/40/30

Sun Nov 19 14:09:05 EST 2017

there is an End Fed Half Wave Antenna group on Facebook
started up by N4LQ. it certainly promotes the EFHW fed with
a 49:1 transformer. Steve loves the EFHW, but really does not
have much love for 9:1 unun antenna. still, it is an interesting
group to monitor.

al ve3gam

--------------------------------------------------
From: "Erik Basilier" <ebasilier at cox.net>
Sent: Sunday, November 19, 2017 3:22 AM
To: "'JT Croteau'" <jt.tobit at gmail.com>; <elecraft at mailman.qth.net>
Subject: Re: [Elecraft] KX3 Field Ant. for 80/40/30

> End-fed antennas have gotten popular lately. When I look closer I see two
> different popular approaches.
>
> The first uses a 9:1 impedance transformer in combination with a wire 
> length
> that is not resonant on any band. The idea is that (assuming there is no
> significant feedline length) you have a medium impedance (450 ohms) on the
> antenna side of the transformer, and because the wire is not resonant so 
> you
> might have an impedance into the wire that is also "medium". By adjusting
> the wire length, you might get pretty close to 450 ohms on one or two 
> bands,
> and with a wide range tuner you can probably get below swr 2:1 for the 
> radio
> PA to see.
>
> The second approach, used by MyAntennas and others seems to use a
> transformer with much higher impedance ratio. One way to construct such a
> transformer would be to cascade two 9:1 units for an effective ratio of
> 81:1. This would mean the wire should present an impedance of 4000 ohms or
> so. Another way would be using a single tranformer with a higher ratio. 
> The
> impedance ratio is the square of the turns ratio. With a turns ratio of 
> 9:1
> you should again get to about 4000 ohms. Somewhere I saw somebody using an
> 8:1 turns ratio for an ideal antenna wire impedance of about 3200 ohms.
> These impedance levels are achieved by using a resonant wire.
>
> I don't know how you arrived at your parameters, but your wire length is 
> too
> close to resonance on 80 and 40. Your transformer ratio wants a 
> non-resonant
> wire, so you might see better results if you shorten the wire 
> significantly
> and keep using the KX3 ATU. Alternatively, you could replace the 
> transformer
> for a much higher impedance ratio, in which case you can probably operate
> with the tuner bypassed at least on 80 and 40 with a well adjusted wire
> length. With this approach you want the wire resonant on each band. It
> should be easy to achieve resonance on 80, 40, 20 and 10. As you double 
> the
> frequency, you are changing the number of half wavelengths covered by the
> wire; the end feedpoint is always at the end of one of these half
> wavelengths, and thus you get the very high impedance that you seek. 30
> meters does not fit as clearly into this scheme. The commercial versions 
> use
> a small coil in the wire located close to the transformer end, and seem to
> be able to achieve a reasoable match for all the bands 80 and up without
> using a tuner. Now if you had placed the feedpoint in the center you would
> not have been able to get this consistency of feedpoint impedance from 
> band
> to band. As I see it, this is a major reason for the popularity of the
> end-fed approach as contrasted to the conventional center-fed approach. 
> Note
> that the 30m coverage of the 80 meter and up design is not replicated if 
> you
> try the same approach with half the wire length. In this case you will 
> need
> a tuner to get reasonable swr on 30.
>
> An important consideration is antenna height. We all know that antennas
> usually work better when placed higher. Looking a bit closer, we can look
> beyond the general installation height and consider the height(s) of the
> antenna part(s) that carry the most current. Antenna modelling may 
> calculate
> the field as resulting from current levels in different individual pieces 
> of
> the wire, and then it makes sense to elevate those portions more than 
> other
> parts of the wire that carry less current. Another reason for this is the
> effect of ground losses. Jim Brown, in his article that he just linked to,
> shows that ground losses get worse the closer a vertical antenna is to
> ground. This makes sense as currents in the lossy soil are caused by
> induction from currents in the antenna. When we look at the current
> distribution within the vertical antenna wire it again makes sense to 
> place
> the part(s) of the wire with high current higher rather than lower. One of
> the simplest portable antennas is a short wire or whip of a quarter wave 
> or
> less. It will have a low feedpoint impedance that can probably be matched
> reasonably without a tuner or with a limited-range tuner. However, with a
> low impedance comes a current maximum at the feedpoint. This often means
> close to the ground, so even with a good set of elevated radials,
> considerable ground losses could be expected. (An actual connection to the
> soil would generally be much worse, unless you bury a lot of wires.) With 
> a
> longer wire (1/2 wavelengh at the lowest band) we can have a very high
> feedpoint impedance, very low feedpoint current, and more elevated
> location(s) of high curent portion(s) of the antenna, for lower ground
> losses.
>
> Any antenna feed point needs to provide two terminals for the feed current
> to flow through a complete circuit. A end-fed designed for medium to high
> feed impedance has small feed current. In practice this means that 
> whatever
> is used as the counterpoise side can be small. The applies with a "medium
> impedance" design as discussed earlier, but it applies even more for a
> resonant end-fed with its tiny feed current. A short piece of wire may be
> used, but often not even that is needed, as the feed return current may 
> flow
> on the transformer and feedline (if used), and even on the radio box. This
> is fine for QRP and maybe even medium power, as the current is small
> relative to the higher current higher up on the wire. However, at medium 
> to
> high power, if matching problems are encountered, or bothersome RF around
> the rig, I would consider a small counterpoise wire to the high impedance
> side of the transformer, or experimenting with the length of the feedline,
> as the length of it matters when "counterpoise current" flows on the 
> outside
> of it. I would not count on the impedance transformer to function as a
> common mode choke for blocking such RF current. You could place a separate
> common mode choke somewhere on the feedline, and move its position as a 
> way
> to adjust the length of the effective counterpoise. Look up Jim's article 
> on
> how to build an effective common mode choke.
>
> 73,
> Erik K7TV
>
> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of JT Croteau
> Sent: Saturday, November 18, 2017 2:34 PM
> To: elecraft at mailman.qth.net
> Subject: [Elecraft] KX3 Field Ant. for 80/40/30
>
> Friends, what would be a good end fed wire setup that will match well with
> the internal KX3 ATU and cover, hopefully, 80, 40, and 30 meters?
>
> I went out to my winter camp site, with two really tall pine trees, and
> tried to experiment with a 9:1 UNUN, 135' radiator, and 35' of RG8X.  It 
> was
> a total disaster.  Best matches were on 20, 17, and 15 meters but only 
> with
> 6.4:1 SWR.  No match at all on the lower bands.
> With my two trees, the radiator made for a perfect inverted-L shape.
>
> What should I try next?  I did try some pruning of the antenna but quickly
> gave up due to the WX conditions.
>
> Thanks
> N1ESE
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