Topband: Shunt feeding the Skyneedle

[Tom W8JI]

You can temporarily use an inductor in series with the cap to extend the 
range. It will not be a good idea for transmitting, but OK for tuning.

----- Original Message ----- 
From: "Carl Braun" <Carl.Braun at lairdtech.com>
To: "Tom W8JI" <w8ji at w8ji.com>
Cc: "160" <topband at contesting.com>
Sent: Tuesday, February 04, 2014 10:27 PM
Subject: Re: Topband: Shunt feeding the Skyneedle


> 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.
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