[VHFcontesting] VHF/UHF SWR meter

[MICHAEL SAPP]

HI All: Years ago N3LL was kind enough to give me a Bird Model 6250 termination milliwatt meter which covers 30-500MHz. That and a few Narda directional couplers provided useful SWR and power measurememts in its spec range as well as relative power measurements through 23cm. I recall swapping in a higher frequency detector diode for awhile to measure relative power up to 3456Mhz. It also served as a "first try" test guardian for the 10mw thermistor head for my old HP430C.  I have never come across a (surplus) Narda directional coupler that did not seem to work as indicated on it's chart plot, although it would be possible to find one with a fried termination resistor on the coupling port. If you have a good directional coupler for the intended frequency range and a known V/UHF MHz power source, you could build your own milliwatt meter (diode, 50 ohm load & microampmeter) and calibrate it per the chart plot on the coupler. 

If you were going to build or test antennas for 902 and higher, an attractive set of instruments would be something like a signal generator and  20 db directional coupler for the frequency range of interest, a frequency counter, and whatever microwave power meter one can find. For example, I was building an Alford slot for 1296MHz last year for the W3KWH beacon project and using such an equipment setup it was possible to immediately determine the antenna resonance frequency, which turned out to be around 1305 MHz (point of lowest return loss using the directional coupler in the reverse in-line position). Once the minimum return loss point was found, the freq counter was used to determine the exact frequency output of the signal generator used as the rf power source.

When building the 4:1 balun for this antenna, the same arrangement was used substiting a 220 ohm 1/4W resistor for the antenna. My first try was too short, having to add 1/4 inch total to the half-wave teflon coax length to get minimum return loss at 1296. Knowing the balun was correct was essential to getting the antenna to work and it removed one uncertainty factor for me

 Some UV protected nylon wire ties were then used to squeeze the slot spacing of the slot antenna to resonance at 1296MHz. Without these tools I could have fiddled around for days verses making it an afternoon project. All told, the dollar investment in these "e-bay" instruments was less than a Bird 43 and a few slugs, while being much more versatile.  Mike WA3TTS