QRP SWR Bridges

[NZ0I]

Modeling has made it apparent that the directional coupler signals will need to be amplified in order for the bridge to operate at low HF power levels (~100 mW). These bridges are more sensitive at VHF frequencies, so it is not yet clear whether amplification will be required for the VHF coupler. SPICE is working to provide believable results for a modeled coupler, showing high reflected levels for impedance mismatches, and near-zero reflected levels for antenna loads near 50 ohms. If I can model the antenna feedpoint as a combination of RLC, then SPICE might provide helpful results for determining the best range of switched C values to use with the matching unit. Just need to figure out the R, L, and C at the feedpoint for a 10-foot length of wire above lossy ground.

To address the need to amplify the forward/reflected power measurement voltages, I've played around with various schemes using op amps working as simple amplifiers, difference amplifiers, and log amplifiers, with limited success. Instead I'm leaning toward a design similar to the one described in this article: http://www.arrl.org/files/file/Product%20Notes/2012%20Handbook/KAUNE.pdf

The actual parts specified in that article are too expensive; the log amps alone are over $12 each. But I think equally-effective alternatives are available. Instead of the AD8307 log amps, I'm thinking of using this part: http://www.digikey.com/products/en?keywords=LTC5507ES6%23TRMPBFCT-ND. The LTC5507 has 20dB of dynamic range, which should work well for 100mW to 2W (13dB) range of the transmitters. This same part will also work at VHF frequencies, if it turns out that amplification is needed in the VHF SWR detector as well.

The same LF398N sample-and-hold amplifiers shown in the article might also be suitable in our application. But they might not be necessary, since in our rather unique application the processor will know ahead of time when (and for how long) a valid signal level will be present at the antenna output, and can time SWR measurements appropriately (e.g., just before the end of each Morse code element).