VHF Impedance Bridge

I've been trying to get into the 147.0 ARNSW repeater, so I can join their homebrew group's net on some Tuesdays of the month. Unfortunately my old FT-415 just can't make the trip barefoot, so I wanted to make an antenna that I could wander up the street with and perhaps get in...

I've been meaning to give the "flower pot antenna" a go, so I wrote to John Bishop VK2ZOI and he was kind enough to send me his articles which include dimensions and all the information needed to make 6 m, 2 m, 70 cm and dual-band 2 m/70 cm antennas. All I needed was a way to assess the VSWR and prove I'd constructed them correctly... I had to build myself some kind of SWR detector for 2 metres.

In the past, I've used my trusty Z-bridge on HF, but at 2 meters it gave me non-zero returns into a known-good dummy load - not a good sign. This project is another Z-bridge, direct from the pages of EMRFD (page 7.24, diagram 7.43). It is claimed to operate into the UHF region, I haven't proved that or really expect it with the sub-optimal resistors I used, however it is perfectly flat to beyond 200 MHz according to my measurements.

Front of Bridge

I threw it together in about 1 hour - most of that time was trying to get the damn BNC plugs to tin, so I could solder them direct to the board. Some filing and lots of heat did the trick eventually. The meter is from KW-TUBES on eBay, I have several left, they are perfect for this kind of project.

Backside of the Bridge

At maximum sensitivity even my "loadstar" signal generator can make a deflection, so QRP use is fine. With the resistors I used it should handle a few watts.

Circuit Diagram

For the 2 metre antenna project it worked perfectly, allowing me to trim my "flower pot" to a pretty good match. However, the new antenna just brought the pagers in even better for my poor FT-415, and didn't help me crack the repeater... More work to be done here, QRO will be needed I suspect. I might have a go at making a helical resonator to null the pager quite close to my QTH, or even a BPF for the band, but that is probably more plumbing than the old HT is actually worth.



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Circuit Diagram Source application/postscript 11.993 kbytes