Re: Topband: Gamma match success
I would caution against using Russian military surplus doorknobs, or Chinese doorknobs, in a tuned circuit without understanding their dielectrics. I tried a couple and was very disappointed. With HEC 7.5kV doorknobs, you would want to make it out of values of 170pF or below. Below 170pF they have NPO characteristics. Tim N3QE On Tue, Oct 16, 2018 at 1:46 PM Herbert Schoenbohm < herbert.schoenb...@gmail.com> wrote: > Unless you use LMR 600 or Andrew heliax a cap made from standard RG-8 or > 213 has its limitations. eBay has a ton of high current fixed caps very > close to the value you need. Some are from Ukraine (Russian military > surplus) and are very inexpensive. You might also consider a home-brewed > sandwich cap made from aluminum plates and Teflon insulation between them. > This method of heavy duty fixed caps seems to be the vogue of many present > amplifiers and tuners. Another alternative is to get a handful of 5KV > doorknob caps at 100pf and add them in parallel or series until you come up > with the value you need. > > Good luck > > Herb Schoenbohm, KV4FZ > > On Mon, Oct 15, 2018 at 5:18 PM Richard (Rick) Karlquist < > rich...@karlquist.com> wrote: > > > > > > > On 10/15/2018 10:43 AM, Tree wrote: > > > > > I replaced the variable cap with a home made cap using RG8. I had one > > > > Sometimes, capacitors made from coax are lossy. I modeled your > > coaxial capacitor using Simsmith (very easy to do) and the Q turns out > > to be 340, if I did it correctly. So the coaxial capacitor gets a clean > > bill of health and probably doesn't have anything to do with > > your broad bandwidth. > > > > Rick N6RK > > _ > > Topband Reflector Archives - http://www.contesting.com/_topband > > > _ > Topband Reflector Archives - http://www.contesting.com/_topband > _ Topband Reflector Archives - http://www.contesting.com/_topband
Re: Topband: Gamma match success
Unless you use LMR 600 or Andrew heliax a cap made from standard RG-8 or 213 has its limitations. eBay has a ton of high current fixed caps very close to the value you need. Some are from Ukraine (Russian military surplus) and are very inexpensive. You might also consider a home-brewed sandwich cap made from aluminum plates and Teflon insulation between them. This method of heavy duty fixed caps seems to be the vogue of many present amplifiers and tuners. Another alternative is to get a handful of 5KV doorknob caps at 100pf and add them in parallel or series until you come up with the value you need. Good luck Herb Schoenbohm, KV4FZ On Mon, Oct 15, 2018 at 5:18 PM Richard (Rick) Karlquist < rich...@karlquist.com> wrote: > > > On 10/15/2018 10:43 AM, Tree wrote: > > > I replaced the variable cap with a home made cap using RG8. I had one > > Sometimes, capacitors made from coax are lossy. I modeled your > coaxial capacitor using Simsmith (very easy to do) and the Q turns out > to be 340, if I did it correctly. So the coaxial capacitor gets a clean > bill of health and probably doesn't have anything to do with > your broad bandwidth. > > Rick N6RK > _ > Topband Reflector Archives - http://www.contesting.com/_topband > _ Topband Reflector Archives - http://www.contesting.com/_topband
Re: Topband: Gamma match success
I usually use the complex impedance as my guide here. What is the resistive component vs the model? If the SWR is flat it is often because the resistive ( R ) portion is too high and dominating the frequency dependence of the X portion. Adding radials can improve that as well as the loss of the match itself. A model of the ideal complex impedance vs ideal ground and lossless match components compared to what you are actually reading will help point you in the right direction. My 70 ft T top Verticals with 45 1 / 4 wave radials, have a resistive component that matches with the model if I make the ground less than average - which is in fact the case here in Central Vermont. Modeling suggests that adding another 20 radials is not going to add much to that equation so I stopped there. I use choke baluns at the feed (bottom) to minimize any coax radiation and adjusted the SWR based on the length of the wires only (keeping the T portion symmetric). The individual Ts have about a 60khz 2:1 bandwidth vs an ideal of about 35 khz. But its all modelable ground losses. I phase 2 of them for end fire to EU and some other interesting patterns depending on phase relationship. The phasing clearly affects the SWR. Ed N1UR _ Topband Reflector Archives - http://www.contesting.com/_topband
