I would like to share my experience thus far with the MFJ-9232 QRP Loop Tuner. I would call this a review, but I’m not going to try and convince you whether you should buy this or not, I’ll just share my experience and you decide for yourself. This is a product that MFJ recently came out with. I’ve been intrigued by loop antennas for quite a while because of their capabilities as a low profile antenna. I’ve been very interested in a 40M loop antenna for a low profile base station antenna at home. Sadly, most of the commercially available ones were out of my price range, and were not very efficient on the 40M band. There’s also the route of building one yourself, which I would like to do eventually. For those not familiar with loop antennas, there’s a good post over at AMRRON, and numerous sources online.
When I received my MFJ-9232 it came packaged in a waterproof vinyl bag with 55 feet of #18 wire and about 10 #10 ring lugs. The first thing I did when I received my tuner was read the instruction manual that came with it, which matches the one available from their website. In this manual they recommend using nothing smaller than a 10 gauge copper wire for your loop conductor and they include a chart of lengths to get good results on each band. I started out with pieces of #10 stranded cut to the lengths in the book, and I had a little luck on 30M through the reverse beacon network, but no real luck on 40M or 20M that day. To be fair, band conditions were really poor that day, but it was still fun.
After this I went and bought 15 feet of 3/8 copper tubing at the hardware store. I double checked the length when I got it home, and hammered the ends flat and drilled a hole through them to attach to the tuner. This was the length to get onto the 30 meter band according to the manual. I had good results with this on the reverse beacon network with decent band conditions, and running 5-15 watts of power from my back deck it produced a distinct T shaped radiation pattern (according to the results from RBN).
Although I had good results on 30 meters, it took a lot of work to get those results. I’ll explain what I mean. With a loop antenna you do not use an antenna tuner, instead you adjust the antenna’s capacitance through adjustable capacitors. At the time I did not have an antenna analyzer, so I was using the method described in the manual of peaking the signal. It is a very hard thing to do because a loop antenna has a very narrow bandwidth, so the sweet spot (low SWR) is very small. The way I would check to see if I was ok was using the reverse power meter on my SGC SG 2020 in CW with about 5W of power.
To complicate things further, your hand will introduce more capacitance to the antenna, so the adjustment changes as soon as you pull your hand away. So after much frustration on 30 meters I purchased a used MFJ-207 antenna analyzer. I picked it up for about $75 and it has made a world of difference in setting up all of my antennas. Once I got the MFJ-207, I trimmed my copper Loop to 12 feet so I could try and make some SSB contacts on 20M. (This was the length per the manual)
I could clearly see the effects my hand was having on the antenna when I was trying to tune it and it was a very painstaking process to get a low SWR. I found that if I actually tuned the antenna to a 3:1 SWR, once I removed my hand it would typically drop below 2:1. I decided at this point I would extend the shaft for the tuning knob and put a bigger knob on to help make finer adjustments. To be fair, once I had it set for a low SWR, it received very well on 20M. I could barely move 1 MHz without picking up another conversation. This was obviously when the band was good. I didn’t have time that evening to try and make any contacts.
It was at this point in my experimentation that I noticed the tuning knob suddenly got a lot stiffer to turn. I took the tuner apart for the first time and realized that something was bound up inside the little variable film capacitor inside. I called MFJ the next day and they had another one coming in the mail right away. Once I received this I simply swapped it out with the original. After that I extended the shaft for the tuning knob with a 1/4″ hose barbed coupling, drilled out on one end to accept the shaft on the capacitor. Once the shaft would fit I epoxied it into the coupling, and put an 8-24 bolt in the other end. I reused a lid from a jar of peanuts I finished awhile ago for the knob.
So far the antenna is a lot easier to tune then it was before. I can usually tune to get about a 1.2 SWR, but I have very high reflective power now. I’m a little perplexed by this, but I’m thinking 12′ is not a good length after all. I’m thinking of sending it back to MFJ and have them take a look at it in case I hooked something up wrong with the capacitor. I still think that the loop antenna concept holds a lot of promise, and I would like to build one for 40 meters. As far as whether or not I would recommend the MFJ-9232….I don’t think I would for portable QRP work. It’s very hard to get tuned and I think in order to build a big enough loop antenna to be efficient on 40 meters with QRP power, it would kind of defeat the purpose of being portable. I still think the loop is more of a low profile base antenna. Therefore, you could buy one of the bigger loop antenna tuners, such as the MFJ-936B, with all the nicer features built in that make it easier to use a loop antenna.