Topband: Shunt feed question

[Joe Subich, W4TV]

On 2019-10-15 4:30 PM, Marty Ray wrote:
> I can obtain a good match using the 65 ft tap point, but my question 
 > is why my analyzer is measuring a change in the feedpoint resistance
 > (real component of R + jX).

Because the *shunt* capacitor creates an L network with the impedance
(R +/- jX) at the bottom of the gamma arm.  Mathematically you are doing
a series to parallel conversion.  For normal gamma operations where the
tap point can be set to achieve close to a 50 Ohm 'real' impedance one
wants the single capacitor *in series* with the gamma arm to "tune out"
the residual (typically) series inductance (+jX).

The alternative is an *OMEGA* match (two capacitors - one shunt from the
gamma arm to ground then another in series with the center conductor of
the coax to the junction of the gamma rod/shunt capacitor).  The omega
match is generally used with an antenna too short that has a high real
"R" at the end of the gamma rod - the shunt capacitor transforms the R
down to 50 Ohms and the series capacitor (or inductor in some cases) 
tunes out the residual X.

73,

    ... Joe, W4TV


On 2019-10-15 4:30 PM, Marty Ray wrote:
> Thanks for the response Herb. I can obtain a good match using the 65 ft tap point, but my question is why my analyzer is measuring a change in the feedpoint resistance (real component of R + jX).
> 
> Regards,
> Marty
> 
>> On Oct 15, 2019, at 3:19 PM, Herbert Schoenbohm <herbert.schoenbohm at gmail.com> wrote:
>>
>> Best to use a 3 or 4 wire cage feed and you will find the match easier.  You should tap the tower at 50 feet and work down till you find the sweet spot.  A 500 to 750 vac variable will take care of any measure inductive component.
>>
>> Herb, KV4FZ
>>
>>> On Tue, Oct 15, 2019 at 3:10 PM Marty Ray <dxcc1 at comcast.net> wrote:
>>> I am shunt feeding a 70 ft Trylon tower with a Tennadyne T12.10-30HD LPDA at 70 ft and a full size 40m rotatable dipole at 79 ft, (the top of the mast is ~85 ft). Both antennas have relays that electrically bond them to the tower when the shunt feed is in use.
>>>
>>> I have tried two shunt tap points, one at 65 feet and another at 45 feet. Using a Rig Expert AA-55 Zoom, the Rs measured a little over 100 ohms on the 65 foot version and 49 ohms on the 45 foot version. In both cases, adding the shunt capacitor caused Rs to drop by approximately 50 percent, (to around 60 ohms and 23 ohms respectively).
>>>
>>> I expected Rs to not change much, if any. I tried a vacuum variable, an air variable and a silver mica. Same result.
>>>
>>> Has anyone seen this happen before?
>>>
>>> Regards,
>>> Marty N9SE