Willis writes: "The input circuit is bypassed by the relay so the only thing in the?path is the relay, coax connectors?and the wires.? I think it is the wires.?"
Craig Buck writes: "I notice high swr when my AL572 is in the antenna circuit - even when turned off.? When I hook the coax directly to the K3, the swr is 1:1. Haven't figured out why except maybe there is a low pass filter built in the amp that is in line all the time.?" I would ask if the relays inside the amps are coaxial relays or open-frame dc relays? I have seen that very common use in HF amps. So it works up to 30-MHz but SWR is high on 50-MHz. Well perhaps you have heard the VHFers refer to HF as the "dc bands"? Manufacturers try to "cheap-out" by using dc relays that are not constant impedance and that works OK on HF, for the most part. But when wavelength becomes shorter so that physical dimensions of relays and "wires" become elements of transmission lines they are no longer 50-ohm impedance. This IS why VHFers use high-quality coaxial relays in their home-built amplifiers with bypass lines of RG-58 or even RG-8. Look inside and see if the relays are "good ole dc relays"? I have a Mirage A1015G 150w 6m amp and guess what the impedance is when in bypass? SWR=1.4 even when I have a 50-ohm termination connected to the output of the amplifier. Connect the termination directly to the coax line that connects K3 to Mirage and SWR=1.0 Fortunately, I have an antenna tuner for 160-6m and I tweak out the mismatch when Mirage is off-line. Input to the Mirage when on-line is 1.5 so I tune the input using the antenna tuner. The antenna SWR= 1.06 so the Mirage is happy without need of the tuner. Suspect those relays! If the bypass is simple buss wire that too will not match well at VHF. 73, Ed - KL7UW, WD2XSH/45 ====================================== BP40IQ 500 KHz - 10-GHz www.kl7uw.com EME: 50-1.1kw?, 144-1.4kw, 432-100w, 1296-60w, 3400-? DUBUS Magazine USA Rep [email protected] ====================================== ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
