Imagine if you scratch-built a dual-band trap dipole out of commonly available hardware store parts, then built a little better one for a friend, then built another for another friend, refining at each step but not over-engineering things. The result would still not be easy to work with, but once up it’s sturdy and functional. That’s about what we’ve got here. The antenna comes all assembled and in four coiled-up bundles of wire between the trap coils and center feedpoint. Ninety feet of copperweld wire with various doohickeys on it is no damn joke when it tries to tangle on you, so be careful. (This is emphatically
not go-bag stuff-able.) When you un-clip each loop of wire, do it outside and be ready to stretch it out, and hang onto it while you carefully unwind it. It does have a really slick feature in that you don’t cut wire to adjust the operating frequencies. Instead, you loosen and slide around these little aluminum blocks to take up or add extra length to each dipole leg, doubling up (or even tripling for the 40 meter part) the wire on that segment. The instructions sheet has equations for how much the frequency shifts per foot you change things, and considering that it takes about 20 minutes to pull things down, adjust, and re-hang before making another SWR check (and I had to do this about four times to get everything tuned), it’s worth the time to do the calculations and measurements before adjusting. BUT the slick part is that if you over-do an adjustment, the wire’s all still there and you just un-do things a little. Clumsy, but very functional.
I had some ambitions to rig this antenna up for NVIS (inspired by this link
http://www.hamuniverse.com/supernvis.html), but didn’t quite have room in the backyard for the full stretched-out length plus the end-poles and their guy lines. So instead I rigged it as an inverted V with the center at 20 feet, because that’s the standard length of 2” PVC pipe and that’s what I had on hand, and ran three 22-gauge wires underneath, borrowing the reflector concept from that NVIS design page.
Back to the tuning, here’re the final SWR plots for this thing on 40 & 80 meters. First on 40:
And on 80 meters:
To give this some perspective, the frequency axes of the two plots cover the General class phone range in each band. For 40 meters, the entire range has good coverage. It could stand being shifted a bit lower, but 2.2 SWR at the worst edge is acceptable. If I ever get into CW, the entire 40 meter band can probably be accommodated by re-adjusting the antenna for slightly lower frequencies by lengthening it by a foot or two. For 80 meters, the center 2/3 of the phone range is nicely covered. I’d heard that trap antennas had narrow working bandwidths, and this confirms it. Still, it is nicely centered and very useable. Switching on the automatic tuner on the transceiver tamed the SWR down to a transmitter-friendly 1.1 across most of the range, getting as high as 2 near the edges of the useable part on 80 meters. We are in business!
As late afternoon faded into early evening last Sunday, transmitting with 100 Watts from my near-Gulfport MS location I was able to easily make several contacts from the Houston area, and as far north as Birmingham. The next evening I was able to talk to another ham in central Florida. As expected, there wasn’t a lot of foreign traffic picked up by this arrangement. Heard one guy in the Dominican Republic have a contact with somebody in the U.S., but the Dominican end was barely understandable. So the bottom line is that this 40/80 NVIS rig gives good regional performance, rejects most signals outside of about 500 miles, adjusts closely enough to 50 ohms on both bands that it could be used without a tuner, and cost only a little more than $100 (including the PVC pipe, but excluding the antenna analyzer & coax). Don't forget to add some kind of RF choke at the feedpoint to keep RF from snaking back down the 50 Ohm coax. MFJ makes a compact little ferrite core model, but there are all sorts of wound coax low-cost designs out there.
There are still some things I’d like to try with this antenna system, but it’s working so well right now that I’m going to leave it alone and enjoy using it for a while. Now that it’s 15’ shorter after adjustment, I’d like to put up two more masts to flatten this out into a full horizontal dipole at 14’. I wonder how that would that change the 80 meter SWR curve? That one 20’ PVC mast could be cut down to two flanged-together 7’ segments which would be far more portable. On the other hand, it would take three of these 14’ masts. Maybe I’ll do it if I get some free time this fall when the weather’s nice. Another thing I’d like to try is re-tuning everything to get the full 40 meter band. I have some friends threatening to learn Morse and it would be nice to jump in there with them. But, like I said, it’s working well for now and I’m going to kick back and get some use out of it until the weather cools.
OK, project successful, and it’s a good antenna for regional communications. On to other matters.
Here’s a link to the manufacturer’s site:
http://www.mfjenterprises.com/Product.php?productid=MFJ-17758 Interestingly, they also make a 40/20 meter version too, which is only about 42 feet long. While it wouldn’t do much on 20 meters if set up for NVIS, it would make a nice compact (if narrow-banded) 40 meter NVIS antenna.
