Charlie,
Consider the following - on my shunt fed tower (100 ft. R25) with multiple
yagis stacked up and down the tower including a 2 el. Cushcraft at 110 ft.
(go to www.w1wqm.org and check the photo gallery, members section, K1RX.
The tower on the left is the shunt fed system). Additionally, I get some
reflective qualities from the tower to the right giving me a gain to the
South and SW directions over my inverted vee at 90 ft. on the right hand
side tower. I connect the gamma rod at the 55 ft. level with a spacing of
5 ft. to the series capacitor. The cap I use is an air variable that
ranges from 300 pf to 1000 pf. Match occurs around the 400 pf area of the
cap.
You did not mention the radials for your antenna? I use elevated radials
(gull wing design) that are 1/4 wave long. Works like a champ.
Regards,
Mark, K1RX
Charlie Ocker <charlie.ocker@tellabs.com>@contesting.com on 12/10/2001
11:19:18 AM
Sent by: owner-towertalk@contesting.com
To: TOWERTALK REFLECTOR <towertalk@contesting.com>
cc:
Subject: [TowerTalk] 160m shunt-fed tower help
Greetings,
I'm looking for some guidance/tips/etc. on my 160m shunt-fed tower
experiment. Tower is 84' of Rohn 45, guyed in two places, guy wires
broken up with insulators. First set of insulators is 5' from torque
arms. For antennas, I have a XM240 at 95', a KT34A at 85', and another
KT34A at 45' (fixed east). I have a couple of inverted vee's hanging
off of sidearms. 80m is at 80', 40m is at 37'.
I've read the info in ON4UN's Low Band DX'ing book, as well as ARRL
Antenna Handbook. The gamma wire is #12 THHN stranded. The gamma arm
is 5' of 1" diameter aluminum tubing. I have a 150~370 pf air variable
capacitor (big honkin' breadslicer) in a wx-proof housing for the series
capacitor. There is a 5' long pvc water pipe mounted at 20', and the
gamma wire slopes from this point to the series cap mounted at base of
tower. The gamma wire spacing is constant from this insulator point to
the gamma tap arm.
Iteration Tap Point Spacing Resonant Freq.
--------- --------- ------- --------------
1 36' 42" 2.6 MHz
2 36' 42" 3.0 MHz *
2 42' 42" 2.8 MHz
3 50' 42" 3.2 MHz **
4 25' 42" unstable ***
5 36' 24" 5.0 MHz
6 34' 60" 2.6 MHz ****
* => Gamma wire spacing constant to bottom of tower, did not slope
from
20' as rest of
interations did.
** => Not a viable solution, as gamma wire passes through horizontal
plane of lower tribander.
*** => Could not find "clean" vswr dip, as freq. of dip varied randomly.
**** => Tap arm moved out to 60", sloped to pvc pipe insulator at 42"
spacing, then sloped from
there to series cap. Ran out of time to lengthen pvc pipe
insulator
to 60" spacing, so
not sure if a solution exists with increased gamma wire spacing
or
not.
I seem to be stuck at 2.6 MHz. I was suprised that both moving tap
point higher up the tower and decreasing gamma wire spacing both
resulted in raising resonant frequency. The highest gamma arm tap point
that I feel would work is about 40', as I don't want to disrupt the
lower tribander. Even here, not sure that 5' spacing won't cause lower
tribander to complain. I feel that there is a workable solution....
from the charts I have seen, I should have enough loading (from the
yagi's) to make the tower electrically close to 90 degrees in length.
If you've followed along so far, I would appreciate any thoughts that
you might have as to how to proceed from here, especially so if you've
gotten your similiarly size/equipped tower to shunt-feed on 160. I'm
aware of the omega-matching technique, where another capacitor is added
from the feedpoint to the tower (and ground). Any feedback relating to
shunt-feeding towers on 160 are welcomed.
73 es hope to cu on topband soon....
Charlie N9CO
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List Sponsored by AN Wireless: AN Wireless handles Rohn tower systems,
Trylon Titan towers, coax, hardline and more. Also check out our self
supporting towers up to 100 feet for under $1500!! http://www.anwireless.com
-----
FAQ on WWW: http://www.contesting.com/FAQ/towertalk
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