I recently had a reader email regarding his low power (QRP) field antenna utilizing the NVIS mode. He commented on his systems lack of performance.
He is using a dipole with 65’4″ legs for both the 40 and the 80 meter band. He is using a bnc-to-binding-post adapter as his “cobra head” with WD-1 field phone wire for antenna wire. His radio system consists of a Yaesu 857 and an AT-100 Pro Auto Tuner. He is trying to use NVIS to fill in the skip zone gaps in his AO.
Here are the problems that I see:
1. Trying to use one home made dipole for 2 bands.
Using the formula 234/f Mhz (for quarter wave length) and assuming the middle of the 80 meter band is 3.75 Mhz, that gives us a length of 62.5′ for each element. So the antenna is now a half-wave dipole and is good-to-go for 80 meters.
For 40 meters that same formula results in a quarter wave length of 32.7 or 33′. He is now trying to use what amounts to a full-wave antenna for the 40 meter band. A full wave antenna is a bear to work with due to the very high impedance at the center feed point which makes them very difficult to match. Nearly impossible to use with any coax.
2. Lack of a matching device at the antenna feed point. Here is a picture of his feed device or “Cobra Head”. (Don’t worry dude, I removed all geo-location data from the pic). By the way, I definitely would not have any loops tied in insulated antenna wire for strain relief. You’ve built in RF chokes. Non-insulated wire – no problem.
When you are working with NVIS, the input impedance changes with the height of the antenna above ground. From as low as 15 ohms near the ground to as high as 120 ohms when the antenna is raised. Depending on the coax such as RG-58 (5O ohms) or RG-8 (75 ohms), you will have mismatches. If you are using this system with regular long-haul comms and the antenna suspended at least 1/4 wave above the ground, it will work just fine.
The tranceiver tuner will take up a lot of the slack, but it will not reduce the losses, it just hides them from the transceiver. When working QRP you need every watt to radiate from the antenna.
Here’s how I would fix the problem.
1. Order solid or braided, non-insulated wire from thewireman.com, measure out the required maximum length for 2, 1/4 wave sections on the longest band you will use and spool each up. Hint: There are very, very, very few resources available for 160 meter DF. Remember, you can always go shorter, but it’s hard to go longer if you don’t have enough wire in your ruck and you’re 100 miles from no-where. Cut (not a physical cut but a measure to length of wire rolled out, the rest still secured on the spool) your antenna length for the freq you will be on, using the above formula. Then spool it back up when you are done.
2. If running in NVIS mode, install a 1:1 current balun like this:
You can find it here: http://www.balundesigns.com/qrp-model-1110-1-1-isolation-choke-balun-1-54-mhz/
While it won’t fix all your problems, but it will clean up your signal.
3. Add a 30′ max length of RG-58.
4. 2 – 50′ sections of 550 paracord with a large bullet type bank fishing sinker tied to one end of each.
5. Measure out the antenna wire from the spools and secure the remainder. Attach each free running end of the wire, as well as the coax, to the balun. Attach a section of 550 cord to the end of each spool, throw the sinkers over a tree limb and host each end up to the height desired and tie the weighted ends off. Move the antenna up and down until you hit the sweet spot with the guy on the other end.
Bottom line: If your going to drop a grand on a high end QRP rig and tuner, you really need to spend some time on the most important part of your comms system, the antenna.