Topband: Shunt feeding the Skyneedle

[Carl Braun]

Thanks for the input Tom

The only variable cap I have is the EF Johnson which is 60-160pf. I have some ham radio stuff at my parents house not the least is a Jennings 1000pf vac variable rated at 5KV or 7.5kv. I was hoping to use that with a 12v motor for QSYing up the band for contesting. I'll have to ask mom to send it to CA in a pkg with some cookies. 

When the gamma arm was at 90' I was able to add 160pf to get a resonance point around 1825 but the resistance was still high at 58-60 and X was 20++. Maybe the big vacuum cap  would bring that R and X down to where it needs to be. 

ON4UNs figure 9-85 on page 9-71 of his third edition shows that a tower that is electrical 110 to 130 degrees should have a tap height around 20 meters and a matching cap of 400pf. That being said it may be a good idea to get the vac variable into service. I would assume I would want to raise the gamma arm back up to 90' as it resonated closer to 1825 than the latest iteration which shows a Fr near
1.977 
 
 
Sent from my iPhone

> On Feb 4, 2014, at 6:40 PM, "Tom W8JI" <w8ji at w8ji.com> wrote:
> 
> Here's what changed though...when I had the gamma arm at 90' with the 14 gauge gamma wire 24" away from the tower I was able to insert my Johnson 60-160pf variable cap in series with the gamma wire to get approx 58-60 ohms at X=20.  The cap was 2/3 meshed at this point.  >>>
> 
> That's the right way. You have to cancel the reatcance of the drop arm to get a good reading. Maybe you need a larger capacitor to hit the bottom of the band? Resistance normally goes up in a case like yours as frequency is drecreased.
> 
> <<<NOW that I've lowered the gamma arm to the 67' level...I insert my variable cap and the antenna resonates at 1.970 MHz with R=36 ohms and X=0. For some odd reason the MFJ SWR reading shows 1.0:1 with this 36 ohm reading and, inside the shack, the Ft1000D shows 1.0:1 swr from 1.988 to 1.950 and a 1.5:1 range of 2.007 to 1.930>>>
> 
> What does more capacitance do?
> 
> <<<<It now appears that the antenna is a bit short but why am I seeing these crazy high resistance readings with no variable cap in line?>>>
> 
> You should see them. The MFJ detector is a 50 ohm bridge. It will overflow and give all kinds of goofy readings when impedance is far away from 50 ohms.
> 
> <<How can I lower the resonant freq without moving the gamma arm up? Increase the spacing of the gamma wire from the tower? Add more radials?>>>
> 
> I would have left it at the top and shorted the wire to the tower at different places until I found the sweet spot. But you have to dip the reactance out to really know what you have.
> 
> <<<
> I was going to build a three conductor wire cage with the wires spaced 10" apart or so once I had an idea where the antenna resonates.  Would a fatter gamma trio drop the resonant freq or just change the capacitance value of the antenna?>>>
> 
> A fatter shunt wire will lower reactance and resistance. You will need more C, and the tuned resistance will be a bit lower.