I just did the following experiment: I coupled a grid-dip oscillator to my feed line far from the tuners and switching relays. I put it a few inches from the feedline equidistant from the conductors. I found that the T network plus DXE balun reduced the signal pickup by about 15 dB over the Matchbox!
That means that both radiation and noise pickup on the feedline are much less with the T than with the Matchbox. At the same time I verified that there was no detectable difference between the strength of received signals coming from the antenna. This is amazing! It means that the T network and balun is significantly better than the Matchbox in this important respect. So much for mythology. On 3/10/2012 1:11 PM, Vic K2VCO wrote: > I've recently been experimenting with tuners. I have a "275 watt" Johnson > Matchbox > (which is actually good for more than a kW on CW in most cases) and a huge T > network > that I just built with a massive edge-wound rotary inductor and capacitors > with air gaps > of about 3/8" (near 10mm). The T network has a DX Engineering "5 kW" rated > 1:1 balun on > the output. > > The antenna is an 88-foot dipole fed with 500 ohm open-wire line and some > 450-ohm ladder > line. > > I can switch between these tuners instantly. On 40 meter received signals I > can detect > absolutely no difference in signal strength. The T network is adjusted for > the least > possible inductance that gives a 1:1 SWR, and the output capacitor is at > maximum (300 pf > air plus 300 pf ceramic padder).. > > There is also no difference in noise level. If one of the tuners provided > better > balance, one would expect that there would be less noise pickup on the > feedline. But I > don't see this. > > One anomalous result: there is a weak unstable carrier that I can hear on the > Matchbox > but not on the T network. I have verified that this is not a birdie, but an > actual > signal. It could be attributed to feedline pickup -- but wouldn't you expect > the > matchbox to be better in this regard? I'm investigating further. > > -- > Vic, K2VCO > Fresno CA > http://www.qsl.net/k2vco/ ______________________________________________________________ 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
