OT: Need Antenna Tuning Help

[Hisashi T Fujinaka]

On Thu, 8 Mar 2012, Tony Estep wrote:

> On Thu, Mar 8, 2012 at 10:54 AM, WILLIS COOKE <[hidden email]> wrote:
>> I wouldn't have mentioned it Jim, except Tony's funny comment muddied the water...
> ===========
> Yeah, there's always some smart-aleck to confuse the issue. But
> seriously, I don't see why MFJ couldn't put a minus sign or something
> on the display. My cheapo Autek gadget (<$100 on ebay) tells you
> series and parallel reactance with sign, equivalent C, equivalent L,
> phase angle, and some other stuff as well as SWR. I'm not an
> MFJ-basher; I have been happy with all the MFJ stuff I have. But in
> this case IMHO they left out some worthwhile functionality.

Look at the different analyzer comparisons and you'll notice that a lot
of them do NOT have the sign of the reactance. My buddy bought the Autek
back in the day because of that reason and it was several hundred
dollars.

I just bought a Rig Expert for the same sort of reasons.

--
Hisashi T Fujinaka - [hidden email]
BSEE(6/86) + BSChem(3/95) + BAEnglish(8/95) + MSCS(8/03) + $2.50 = latte
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[Guy, K2AV]

Below...

> On   Thursday, Mar 8, 2012, at  Thursday, 12:14 PM, Gary Hinson wrote:
>
> > ... and thank YOU Jim for your encouraging words.  I do have the space
> for
> > several quadrillion radials but not the energy to cut them all into the
> > ground below sheep/cattle/deer foot level.  So I'm waiting for the
> postman
> > to deliver some cores from Amidon, and I'll give an FCP a go.
> >
> > One thing is not v clear on W0UCE's site is the antenna: he shows an
> > inverted L coupled directly in - it looks v simple but my inv-L will be
> > different so aside from trimming the length and maybe configuration of
> the
> > L, are there any other tricks to achieving system resonance?    I have
> tall
> > fir trees to hang the antenna from, so mine will be close to a quarter
> wave
> > vertical I hope.
> >
> > 73
> > Gary  ZL2iFB   www.G4iFB.com

Hopefully a short simple addition to the discussion. Jim is using the 80m
version of the "simple" arrangement: FCP plus required isolation
transformer (IsoT) plus quarter-wave-ish radiator that is pruned to center
the best impedance range at the desired operating frequency, and has the
mechanical simplicity of a balun in the middle of an inverted vee.

The FCP, being 5/16 on its only band (not multiband) is quite capacitively
reactive.  The IsoT has quite a bit of leftover inductive reactance on its
band.  The parts in the IsoT were chosen for efficiency, and in addition,
under the antenna load, to have an inductive reactance that is close to the
FCP, with the opposite sign.  The turns count on both the 160 and 80m
versions are specifically picked to allow you to put up a
close-to-quarter-wave radiator that can be pruned to a zero reactance and
an R in the 50-ish range as seen ON THE SHACK SIDE of the IsoT.

Most are putting up inverted L's as radiators, for
residential-plot-specific reasons, where trees are, etc.  But you can put
up any radiator you want. The different X and R curves for various
"typical" 1/4 wave inverted L's and other radiators mean that sometimes you
may have to start adding things to the IsoT to tune R and X. This gets into
understanding tuning networks. There are some remarkably useful complex
configurations using IsoT's and FCP's, but you won't work them just reading
SWR, and you'll need to be looking at them as complex LCR networks.

If you put the behavior of the IsoT under load into a model, you have to
use an LC network plus a transformer widget to describe it. So in truth the
antenna per se -- the 1/4-wave-ish radiator over the FCP -- is NOT a
resonant antenna. You are using a required isolation device with INDUCTIVE
non-resonant network characteristics that have been picked to be the
opposite of typical L-wire/FCP CAPACITIVE non-resonant characteristics. For
many, just pruning the L gets it close enough. Others have lengthened the L
to make it inductive and put a transmitting cap in series.

The details are at http://www.w0uce.net/K2AVantennas.html

 73, Guy.
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