Topband: Looking to improve TX antenna Efficiency

[Guy Olinger K2AV]

We need to put gorilla detectors in SWR meters.

Loss in the ground counterpole will be the thousand pound gorilla in the
room.  Heavily bent antennas as you describe lower the radiation resistance
significantly, which makes the resistance of the counterpole even more
critical than for a pure 125 foot vertical.  Doing stuff in the air with
the wire is unlikely to make significant improvements.  You will have to
deal with the gorilla somehow.

I have had people describe a system with family resemblance to yours tell
me that the feed Z was 45 to 50 ohms.  With an effective radial system, you
should be having to match some thing like 15-20 ohms, perhaps less, and it
should be fairly narrow.  You should need one of those low Z ununs to feed
a low, bent wire, unless you've gone long on the wire to raise Z and be
content with a lot of NVIS.

A friend of mine with a "fan" vertical, kept adding 125 foot radials made
from insulated stranded #18 (he had an inexhaustible supply) on the ground
until he quit lowering Z at NINETY of them.  At that point it was a 17 ohm
feed with a series coil made from 1/4 inch copper tubing.  He had to use a
unun to step it up to 50 ohms for the trek to the shack.  It worked very
well.  Top ten USA finishes in 160 meter contests.

With that much current at the ground, getting dense and uniform all around
is worth a lot of radiated energy.  Irregular, miscellaneous short radials
in/on the ground are just about always serious losers.

With a 160 antenna system, the FIRST thing you plan is where and how to
deal with radials or counterpoise correctly, which could be quite the
difficult exercise, and ONLY THEN start worrying about the wire.  The
payoff for care to that is huge.

73, Guy.

On Tue, Nov 20, 2012 at 12:23 AM, Steven Raas <sjraas at gmail.com> wrote:

> Last 'Season' was my 1st real dive into 160m. Running a 'InvL' ( which
> I will describe lower in detail ) and 100w, I managed to pull 19 DXCC
> and 48 states cfmd. I am in looking into ways to improve my TX antenna
> efficiency, for this season w/o getting to crazy . (rx antennas are in
> the works and a different topic )
>
> Current TX antenna is as follows: (Tuned in shack / not self reasonant )
>
> Main Element: 32 feet vertical - 90 deg bend ( the 'L' ) which runs
> 43' horzontally pointing north. Then, another 90 deg bend, and
> ascending 26' in length ( from 30' up to about 11' above ground
> running from west to east. ), then..you guessed it.. another 90 deg
> bend that now runs to the south flat top for 27' ( @ 11' up )
>
> Ground radials qty 24. Min length 22' - max length 62' ( from az 330
> deg - 70 deg with the longer radials pointed from 35-60 deg az)
>
> The FCP idea is not practicle for me @ this point in time.
>
> My Thought.. add a Coil @ the 30' point of the main vertical run made
> from perhaps 1/8th inch copper tubing ( refrigerant line ) in a fairly
> large diamater ( 16-20" ), perhaps 14-20 turns even spaced, using
> perhaps drilled lexan as spacers.. Try extending the vertical run
> another 8-10' or so.. and then the rest of the antenna as is. Going
> above 40' vertical is not an option @ this point in time either. ( yes
> i know the more vertical the better its just a no-can-do for now)
>
> At the moment the antenna's best 'swr' is at roughtly 1818 khz @ 2:1
> there is no resonant point that I can find within the 160m band. ( no
> antenna anyalizer either - Mabey santa will provide ) My best guess is
> that in its current configuration... the antenna is probabally in the
> 8-12 ohm area.. I have nothing solid to back that up however.
>
> At the feed point I have a coaxial choke, 21 turns on a 8" form, the
> cable is similar to RG213 , with the exception the center conductor is
> solid. ( its actually 50 ohm quad shielded cable i got form a place I
> used to work, HP node cluster inter connect cable. .405" & blue in
> color,  i cant remember the vf on it but i did spec it out a while
> back, and its ok for HF use. )
>
> Or, tuning the antenna @ the base with a variable cap. of  sufficent
> size, or perhaps even both?
>
> I know there is NO magic that can come..im just trying to improve on
> what I have and can do @ this point in time.
>
> Suggestions, thoughts,  flames and raised brows are all welcome.
>
> Steve Raas
> N2JDQ
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>