Cheap SWR/Wattmeter for 630m

Discussion in 'The Low Bands - 630/2200 Meters - VLF' started by N0GMT, Mar 13, 2019.

ad: L-HROutlet
ad: l-rl
ad: MessiPaoloni-1
ad: L-MFJ
ad: Left-2
ad: Subscribe
ad: Left-3
  1. N0GMT

    N0GMT Ham Member QRZ Page

    Here is an easy and inexpensive way to have a SWR/wattmeter for your 630 meter station. I needed something to monitor my output and make sure the antenna was happy. Didn’t have a suitable slug for the Bird, in fact, I don’t think one exists. So, I was wandering through a hamfest, and spied a MFJ-860 dual-needle SWR meter. I really like the dual-needle meters, you can instantly see if there is a problem with your SWR. Surely, it couldn’t be too hard to modify the thing to work on 630 meters? I bought it.


    It sat in the shack for a while until I got the necessary “roundtuit” and wanted to see what it would do. I was thinking I might need to try and add some turns to the toroidal sense inductor, which might be tricky. But then I fired up the WA3ETD 630 meter transverter into a dummy load, with the SWR meter in series. I knew how much power the transverter should be putting out. When I hit the switch, I was pleased to see that the meter indicated just about half as much power as I was actually producing. A quick look at the schematic showed there were four pots that adjusted the forward/reverse and high/low power readings.


    Well, how do I re-calibrate this thing for 630 meters, when I don’t have anything else that works on these frequencies? It’s easy to do, but you will need an oscilloscope, a signal generator, and a little help from George, M1GEO. On his website, he has a nifty calculator that will take your peak-to-peak RF voltage into a 50 ohm load, and tell you the power in watts. You can find this, and much other good stuff, at: https://www.george-smart.co.uk/calculators/power_calculator/

    I played around with the numbers to find a power level that was about mid-range on the meter scales. Had the signal generator set to 4472KHz, the usual frequency out of my transceiver, and connected to the transceiver connection on the transverter. Need the signal generator to provide a continuously variable input signal. Adjust the signal generator output to give the desired P-P RF voltage, measured with the scope probe right on the transverter output. Adjust the forward low power first, (R6)then reverse the coax connectors on the MFJ-860 to adjust the reflected level (R3). Repeat at a higher power setting, forward,(R5) swap the cables, and reflected(R4).


    And there you have it. Is it perfectly calibrated? I doubt it. But, for $60 or less, (think I paid $45) it’s a good way to be able to monitor your output and SWR on the low band, without tying up any more expensive test equipment.



    Rob
    N0GMT
     

Share This Page