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" <w...@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. _________________ Topband Reflector Archives - http://www.contesting.com/_topband