Re: Topband: alternative feedpoint capacitor
My top-loaded vertical needs a series 260pF capacitor at the feedpoint to match the 53 + j335 impedance (1.825 MHz) to my coax feed. I have a few different air-variables I could use, but I was wondering what sort of fixed capacitors would be suitable in this application. Max power is 1500w. I have doorknob caps but they don't have current ratings marked on them. Steve, You had some suggestions on capacitor types. Finding the actual voltage and current on a series capacitor like this is pretty easy. Voltage is the square root of the power times reactance. That is RMS, so you have to consider the peak plus a safety factor. So you have the square root of 1500*335 or 708 volts RMS across the cap. That's about 1kV peak. 2kV would give you good safety factor. Current is just square root of the power over the load resistance. sqrt of 1500/ 53 = 5.32 amps. Exceeding voltage is an instant failure because it will arc. You want good headroom. Current failures are heat and take a little time, so the heat averages over a short period of time. A physically large part can be somewhat abused in Ham duty for current. 73 Tom _ Topband Reflector Archives - http://www.contesting.com/_topband
Re: Topband: T88 opportunity
Jon, There are ready-made place for TB , with TX antennas . It is necessary to organize the good RX only . I used VK5ZVC coaxial loop and preamplifier AMECO PT- 3 http://www.qsl.net/ra0ff/t88ow/ 73! de Eugene RA0FF, ex.T88OW http://www.qsl.net/ra0ff/ Четверг, 6 августа 2015, 7:35 -04:00 от Jon Zaimes j...@verizon.net: Still needing T88 on 160, I noticed this this: http://www.palauislandhome.com/index.html If a ham lands the job, I'll mail some wire for Topband :) 73/Jon AA1K _ Topband Reflector Archives - http://www.contesting.com/_topband _ Topband Reflector Archives - http://www.contesting.com/_topband
Re: Topband: alternative feedpoint capacitor
Thanks for the cap suggestions everyone, and for the V and I calculations Tom. Ideally I'd like the 300pF (or so) vac. variable and so will watch for a decent deal on one. In the meantime, I will parallel four 50pF doorknobs (7500v and 1.8A at 1 MHz) and single 100pF variable with .05 plate spacing (which I think is good for 2kv). With the current divided 5 ways, I should be well within ratings. I may try 5x 50pF doorknobs just to see how close I am for matching given the +/- 10% spec on the caps. 73, Steve KK7UV --- This email has been checked for viruses by Avast antivirus software. https://www.avast.com/antivirus _ Topband Reflector Archives - http://www.contesting.com/_topband
Topband: T88 opportunity
Still needing T88 on 160, I noticed this this: http://www.palauislandhome.com/index.html If a ham lands the job, I'll mail some wire for Topband :) 73/Jon AA1K _ Topband Reflector Archives - http://www.contesting.com/_topband
