TopBand: 160M Shunt Fed Towers
Charles R Constantine
kr6c@juno.com
Wed, 31 Mar 1999 13:17:48 PDT
Hi All...
Well I dont claim to be an expert and am always looking for information
on how to improve an antenna system and maybe someone is in the same
"little pistol" category as myself so here goes.
I live on a city lot about 3 miles from Earl, K6SE (he can sure make that
20dB attenuator in my RX get warm hihi!) in the beautiful Mojave desert.
I got the bug for 160 last October and set out to look for that 160
antenna that would fit in a 70x30' yard...you can stop laughing now.
Nothing seemed to look promising so I turned to Shunt feeding my tower.
The tower is a 30' crank-up with a 15' mast. It has a Cushcraft A3WS(3 el
beam w/30m kit) at 35' and a F-12 C4SXL at 40' and the last 3' of mast
above that.
Not knowing about the loading effects of the F-12 with isolated elements
I figured this puppy is going to be a problem to tune and probably
radiate like my previous HF6V on 160M. I also decided to shunt feed it
for 80M, that way I have beams on 10,12,15,17,20,40M, a rotatable dipole
on 30 and vert on 75/80/160M on a city lot, in a 24 sq ft area (housing
tracks are lousy but beats the condo I moved from!) The following is
what I currently use.
| - 43'
|
-------------------------------------
c4sxl at 40'
|
|
--------------------------
a3ws at 35'
|
160 shunt arm |
----------------|-|
towertop at 30'
| | |
| ---- 5' ----- | |
| | |
| | |
| | |_____ 80 meter
shunt at 20'
| | | |
| | | |
| | |---4'---|
| | | |
~ ~ ~
| | | |
| | | |
120pf === | | === ~120pf
| | | |
|---------------------------|--(ctr cnd wire)--|
choke balun------rg213 to rig 25'
| | |
|
~200pf ==== Omega| | |---(shield wire)-|
|_________|_|____ |
|
Gnd (gnd is 1 8' rod at
base and 3 60' #12 wires around base. cant do much else cuz 1/2 yard is
cement and only a 25sqft patch of grass to west) Shunt wires are rg-6
coax with ctr cnd gnded to shield.
Starting from the first DXcontest this season (oct98?) I have worked 34
countries including T30, BV, 3DA0, UA,VE1, a bunch of carribean and 46
states (need a couple W1's). It seems to radiate but S9 constant noise
makes it hard to hear them.
The 2:1 BW on 160 is 30kHz and 2.5:1 is 40kHz. and at center freq hits
1:1 Easy to change freq by adjusting the Omega cap on 160 of the series
cap on 80. Guess I need some 1rpm dc motors to remotly tune them.
I only need 1 feedline and switching from 80 to 160 is there by nature of
which circuit is resonant.
I used my mfj-204b antenna bridge and connected the end of the 160 shunt
to the center cond and gnd to gnd (bypassing caps) and found what looked
to be a resonance at 4.2mHz at ~ 1000Ohms (which would make sence with
wire being at the HV point of the radiator) and a smaller dip abt 2.1mHz.
So im not sure what the real resonance of the structure is or if its
working like an 1/8 or 1/16 wave antenna on 160 but I tuned it and it
radiates!
Carl had mentioned not to space the shunt wires more than 24" from tower
due to shunt wire becoming the main radiator. Is this really the case?
I thought the gamma match was basically a short length if parallel line
shorted at its end and looks inductive. Even with spacing of 5' the
Gamma wire is well within .01 wavelengths spacing and dont know why it
would tend to be the main radiator? Since my shunt wire is very short I
found spacing the wire away from the tower raised the resistance and made
it easier to match to coax.
Any ideas on improving the system? I have thought about grounding the
elements on the F-12 antenna to increase loading but im not sure what
effect it will have on the F-12 tuning and also am not sure if the A3WS
is actually setting the top-hat effect and if it would even see the F-12
antenna? I drool everytime I pass Earls towers standing so proud in the
air and actually laugh at times when I look at mine. But again...sure
beats the end fed wire qrp operations from my old condo!
73 es tnx fer the BW
Chuck Constantine KR6C
<Added by N6TR: Have you looked into the loss of 3 miles of hardline? :-)
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