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Cross-band Repeat and a Tabletop Antenna

posted Mar 23, 2012, 12:18 PM by Charles Boling   [ updated Mar 23, 2012, 7:11 PM ]

I know, I know; there's nothing new about either cross-band repeat capability in radios or the construction of cheap ground-plane antennas.  This is, however, the first time I've made practical use of either.

DR-590 Ctrl Head showing battery
Last night, I was home watching 7 of the kids while my wife was out.  Not being able (or inclined) to do any work requiring serious concentration, I decided it was finally time to  program the memories of the old Alinco DR-590 that I bought from K7ICY last year, so I could give it some more serious use.  Having done that, I next turned my attention to the cross-band repeat functionality that I enabled at the same time I replaced the battery so that the memories could work.

Cross-band repeat can prove useful in several scenarios, but the one I was most immediately interested in (and was in a prime position to test) was providing a short-range link for use around the house.   Where I live, I am unable to hit the W7DG repeater without the use of an outdoor antenna with a bit of gain.  Unlike people in town, I can't just walk around with my HT and talk on the repeater.  With the Alinco providing the "big gun" connection to the repeater and a low-power 70cm link, though, I could use my HT on 70cm and be mobile!

The DR-590 doesn't have a built-in duplexer, but instead has separate coaxial cables for UHF and VHF.  This is a good thing if you want to use it with separate antennas, but if you want to use the same antenna for both bands, it means buying a $25-75 duplexer.  Looking around the office, I didn't see a spare antenna.  I did, however, recall seeing a salvaged SO-238 connector in my collection, and a scrap of 10AWG Romex in my garage that might make a cute little 70cm tabletop antenna.I considered checking the 70cm SWR on my 80m dipole to see if I could use that (at 450 MHz that system's lossy enough that it "matches" many frequencies, and I wasn't looking for effiency, since it was for short-range use -- I considered the loss a feature) but I thought that I'd rather have a little antenna inside the house where perhaps even less of its power would radiate far enough away to bug anyone else.  The extra antenna would become an asset -- useful someplace besides home, and I'd been wanting to build one for a while anyway.  Besides, building the antenna it would give me something to involve the older kids in once their little buddies were tucked in bed.  That's me -- always thinking of the children.  ;-)
Construction was very straightforward.  After peeking on the Internet (why bother to calculate lengths when you can see what someone else did?  Dividing 19 by 3 is sooo hard...) at a couple of designs, I went out to the garage, found that my scrap was a happy 28" long, folded the black wire into quarters and cut it so that I'd have 4 legs a little less than 7" long, and whacked about 6-1/2" off the red wire for the top element.  After stripping the insulation from one end of each, I carried them back to the workbench, called the kids, and got out the soldering gun.  Then the butane torch.  Acetylene anyone?  It was tough to solder so much metal properly without taking so long that it melted the insulation on the SO-238.  It melted it a little and loosened the center sleeve so that it wiggles, but it doesn't fall out or move off-center, so it was good enough for the purpose.  This is a quick-n-dirty antenna, so I didn't go to a lot of effort to straighten the elements, either.  I did add an Ell to get the cable out of the way so it would stand up better.

I tested SWR and it appeared to be a reasonably flat 3:1 over the top 2/3 of the band, only raising above that below 430MHz.  My antenna analyzer only works to 180MHz, so I didn't bother to test more thoroughly, but I suspect that the impedence is a bit low;  I might try bending the legs a bit more vertical (at the expense of stability), or even play with coax length to see if I can get it to show the radio 2:1 or better.  Again, I'm not looking for the world's most efficient antenna system -- if the radio's happy, I'm happy!

By this time, Michele was home, but I wasn't ready to quit for the night -- do all the work and not get to play with it?  After a brief test of the UHF link (hard to miss when your radios are 3ft from each other!) I tuned the VHF side to the .260 repeater and listened to myself talk a couple of times.  WA7RPM was monitoring the repeater as he often does, and jumped in to see what I was up to.  We had a nice QSO which gave me a much better feel for real-life operation of the link.

The Alinco doesn't seem to allow you to set a band priority (I'd like to set the UHF receive as the priority) so I had to wait for the repeater tail to clear and the Alinco to drop before I could transmit and be heard.  Since I can often hear the repeater on my HT (usually not very well, but readable) I'd like it if the radio supported one-way repeat like I know some of the Kenwoods do, where it would ONLY listen on 70cm.  This would do two things: (1) reduce wear on the radio (and unneeded RF emission) by ONLY helping me get out, rather than parroting everything that comes off the repeater; and (2) allow me to instantly trigger it, so that I don't have to wait for the tail to clear.  Otherwise, I'm afraid it will be like it was when I had the "busy channel lockout" feature on my Wouxun enabled: nobody let the repeater drop long enough for me to get a word in edgewise.  Like Echolink-enabled repeaters, dealing with a cross-bander (or most other linked repeaters) requires a bit of cooperation from all parties to work well.

That cooperation is needed is also true to ensure that your radio stays legal; lacking an electronic IDer, it's necessary for someone else on the main repeater to periodically acknowledge your repeater with your callsign, so that it's heard on its 70cm output.  This will keep you unambiguously within the letter of the law, even if you're standing 3' away IDing on the same frequency with an HT that is probably reaching further that the other radio; if they are separate transmitters, then each should ID itself so that it's impossible for someone to monitor your transmission long-term without being able to hear your callsign.  (Since this radio does allow it to be used normally while in repeat mode, I can also periodically visit it and ID on 70cm myself.)

Overall, I'm pleased with the results of the experiment; everything worked as planned, I got a little experience at operating in that mode so that I'll be better prepared to make use of that tool if it is called for in an emergency or other public-serving function, and I know that the next time the need arises, I can leave my "man cave" for a few moments without missing part of the net, or far more tragic, my wife's attempt to contact me when she's out. (She is still uncomfortable enough with the radio that I don't wish to do anything to discourage her!)