The rig in both cases was a (tr)uSDX and the antenna was a 17-foot whip (MFJ-1979) mounted to the bike's luggage rack, along with one to four 1/4-wave radials on the ground. Or none at all, just using the bike's frame as RF ground.
I also had a RigExpert AA-55 Zoom along with me because I wanted to see the effect of adding radials. And, I also wanted to see if it was feasible to use this antenna on 30 meters.
But the main mission was to determine which tuner would be best to take to the field.
I've always been a bit hesitant to use autotuners with rigs that operate at full power during a tune cycle. This is not the case with tuners built into rigs (or otherwise integrated into the rig's functionality) such as the KX2, KX3, etc. but the (tr)uSDX offers no easy way of reducing power during tuning.
An advantage of manual tuners, especially those with sharp tuning characteristics like the ZM-2 or the TR-45L's optional tuner, is that a very close tuning solution can usually be made without even transmitting, just by tuning for maximum receive noise.
The 90-minute POTA activation consisted of 30 minutes on-the-air and an hour tinkering with the antenna and tuners.
When I got home I decided to make some terminations and put numbers to the tuners. I had plenty of metal-film resistors and dozens of BNC connectors, so I soldered up 6 of them to simulate various SWR's that a tuner might see.
I terminated each tuner with each termination, then tuned for minimum SWR. Then the tuner was attached to the nanoVNA to measure its insertion loss, then to the AA-55 Zoom to measure the 1.5:1 bandwidth of the tuning solution.
The following results were obtained:
With few exceptions, each tuner performs well. Note that this test was made only on the 20 meter band.
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Interesting. But real antennas are often reactive and a 3:1 SWR doesn't necessarily mean 150 ohms or 16.67 ohms; it might mean an antenna impedance of 30 + j40 for example. It would be interesting to see loss figures when those two tuners are asked to match moderately (or not so moderately) reactive antenna impedances. And I know it would be a lot of work, too.
ReplyDeleteDavid VE7EZM and AF7BZ
Hi David,
DeleteYes, that's true. Prior to the above measurements I did another little experiment - I hooked up the items in this order:
IC-705 -> ATU-10 autotuner -> ZM-2 manual tuner
Upon keying the IC-705, the ATU-10 initiated a tune cycle. Then I mis-adjusted one of the caps on the ZM-2 until the ATU-10 went into another tune cycle. I then measured the complex Z of the ZM-2 at the point where the ATU-10 couldn't successfully get a 2:1 (or lower) match.
But as you mention, this is not necessarily the limit of what an ATU-10 could match - it's only the limit at that specific complex Z. And whatever that Z was, there are an almost infinite number of ways to create it with combinations of R and X.
So I fell back on the method used above which is also used in at least one QST review of various tuners.
73,
John AE5X
Hey John, you can reduce the power out on the (tr) uSDX if you reduce the supply voltage. I don't know how much power the ATU-10 needs to do its thing but if you power the (tr) uSDX off of the USB port it will put out about 500mW. If that isn't enough you can temporarily power it from a 9V battery to tune the antenna and then switch to a higher voltage battery to operate. Another idea is a little kit from QRP Guys that they call the LED Tuning Indicator (https://qrpguys.com/tuning-indicator). You can install this between the rig and the ATU. In the TUNE position most of the power goes to the resistive bridge and the rig sees 50 ohms. The tuner should still see a sufficient amount of power to allow it to do its thing. Once the antenna is matched you can switch it back to OPERATE and bypass the bridge. Cheers de VE3WMB
DeleteThanks Michael,
DeleteI had not heard of the tuning indicator before. And I knew, but had forgotten, about being able to power the (tr)uSDX from the USB port. If conditions keep improving, it might be fun to make an activation at the 1/2-watt level.
In truth, I'm probably being overly-paranoid in worrying about damage to the rig. I've used autotuners before, at full output from the radio, and never had any problems.
73,
John AE5X