Topband: Shunt Feed help

[Gene Smar]

Bob:

     I'm guessing that the voltage on the 20M director is due to QRO
operation on 160M.  I recall reading an email from Natan, or at least he was
mentioned as the originator of the advice, here on TT maybe 15 years ago
that the C31XR antenna could have all the parasitic elements shorted to the
boom without adverse effects.  Something about the middle of the parasitics
being a voltage minimum.  I shorted the parasitics on my Skyhawk tribander
as the feedpoint is freakishly similar to F12's design to use it as top
loading for my shunt-fed tower on 160M.

     If you're interested, you can see a photo of the shorting assembly on
my Skyhawk at
http://photobucket.com/gallery/user/ersmar/media/bWVkaWFJZDoxMzI5NDU3Mg==/?r
ef=1 .  I fastened a 3/4 X 5 inch piece of aluminum flashing to the element
with a #10 SS self-tapping screw right through the center PVC insulator.  I
used a SS hose clamp to fasten the other end to the boom, using a small
amount of NoAlOx on the joint.  Then I taped the entire hose clamp.  Both
the Yagi and the shunt-fed tower seem to operate OK AFAICT.

     YMMV.

73 de
Gene Smar  AD3F



-----Original Message-----
From: Topband [mailto:topband-bounces at contesting.com] On Behalf Of Marc
Wullaert ON4MA
Sent: Tuesday, October 10, 2017 1:43 PM
To: bob burton <n4pqx at yahoo.com>; topband at contesting.com
Subject: Re: Topband: Shunt Feed help

Hi BoB,

be careful with the c31xr as the 20m reflector is on the far end. maximum
voltage present there and mine burned down at the isolator while my
freestanding tower (24m) with c31xr and 5m higher  a 40cd2 40m 2element beam
where shuntfed on 160m.

Best way is to ground the element or look for a better isolator

73

Marc ON4MA
https://www.facebook.com/marc.wullaert.5
----- Original Message -----
From: "bob burton via Topband" <topband at contesting.com>
To: <topband at contesting.com>
Sent: Tuesday, October 10, 2017 2:22 PM
Subject: Topband: Shunt Feed help


> Looking for some shunt feed wisdom from the list. Trying to shunt feed 25G

> tower ( 8-10ft sticks, 8ft flat top, base 18" out of the ground = total 
> tower 89.5 ft) has a F12 C31XR just above the thrust bearing (@90ft) with 
> 6ft 2" mast above the tribander with a F12 D140 dipole (@96ft). Based on 
> various archive reading I can't get a good sense of how much top loading I

> may have (some say only the boom, others say much more). That said I've 
> first tried a cage wire ( four 14ga wires spaced 6" apart) all the way at 
> the top of the tower spaced about 2ft out. Trying to dip this with an 
> Autek RF-1 yielded about 1.15 Mhz which didn't seem right. Cage was taken 
> down and tried just a single wire at a lower tap height of 60ft. Strangely

> enough I get about the same frequency but the Z dropped. Moving farther 
> down to 50ft again about the same frequency but again Z dropped. I'm 
> assuming the frequency readings are completely off and I'm getting 
> overload from nearby AM broadcast as I am surrounded by them. I'm unsure 
> now if I should continue trying tap points farther down the tower just 
> looking for the best Z or use some other approach. If anyone has 
> experience with how much top loading a C31XR could represent I could use 
> the graphs in ON4UNs book to perhaps get close, but I'm unsure what the 
> loading looks like and found no real answers searching the archives. Any 
> help appreciated to help eliminate many more trips on the tower looking 
> for the nearest 50 ohm spot. Tnx 73's Bob - N4PQX
> _________________
> Topband Reflector Archives - http://www.contesting.com/_topband 


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