This is the fourth part of my emergency communications system. The system I am building is intended to be modular. When complete I will have a VHF/UHF station, an HF station, a battery box, an antenna system, and a computer system. Having a bunch of hardware and the power to run it is all fine and good, but if your antennas aren’t capable of getting the signal out all of those radios are useless.
Like with my go kits, my first priority was to have a good VHF/UHF antenna system. A magmount on the roof of your car is OK, but for local line-of-sight communications the gain and height of your antenna are very important. I also wanted something that I could easily transport and erect by myself.
After some investigations I decided to aim for a simple lightweight mast system that I could mount to the trailer hitch on my car. The base for this setup is a hitch mount flagpole mount. For the mast I chose the MFJ 1904H. This mast solves a lot of potential problems for a portable mast system: it is 5 feet long when collapsed making it easily transportable in my car, it is non conductive and will not interfere with any antennas mounted on it, and despite being made of fiberglass it is fairly sturdy (each tube wall is 1/8″ thick). I don’t intend to heavily load this mast so this should serve my needs well. In order to achieve a tight fit between the flagpole mount and the mast I used a section of 2″ PVC pipe as a spacer. When fully extended the top of the mast is about 21 feet high.
For the VHF/UHF antenna I chose the Two Way Electronix Dual Band Slim Jim. As someone who uses a J-Pole antenna as my base VHF antenna, I am very familiar with how well these antennas perform. The Slim Jim is a J-Pole made from 450 Ohm ladder line so that it can be rolled up for easy storage and transport. I mount the antenna to the mast using a nylon bolt and wingnut the passes through the insulation at the top of the antenna and a hole drilled in the top fiberglass section. This setup is very light weight and has virtually no wind loading which makes guying the mast unnecessary (under average wind conditions).
For HF I did considerable research regarding what type of antenna is appropriate for emergency communications. A lot of ham radio is focused on making contacts at great distances (DX). This necessitates a low angle of radiation from the antennas being used. For a dipole this means that the antenna should be at least a half wavelength above ground. The most common HF EMCOMM bands are 40 & 80 meters (one half wavelength on 40 meters is about 66 feet, double that for 80 meters). For EMCOMM purposes, however, we only want to communicate within a couple hundred mile radius of our location. This requires a Near Vertical Incident Skywave (NVIS) propagation path. It turns out that this makes our lives a lot easier since a dipole can be used for NVIS when it is mounted much lower than it would typically be. Instead of trying to get a dipole very high, mounting it at 15 feet is ideal for this application. Another benefit of this approach is that at this height the dipole loses almost all of its directionality and is essentially omnidirectional.
In an effort to maximize portability and reduce both setup time the footprint of the EMCOMM station I decided to try MFJ’s hamstick dipoles (MFJ-2240, MFJ-2275). These are loaded antennas that use a base section coil wound on a fiberglass rod with a stainless steel whip on the top. While a small loaded antenna will not have the efficiency, or the bandwidth of a full size antenna, it is considerably smaller (15 feet vs 66 & 133 feet). The hamstick dipoles actually perform fairly well. I was able to tune them for the digital portion of the bands and using the antenna tuner in my Icom 703 I can tune either dipole across their entire respective band. The dipoles are also fairly light weight and my fiberglass mast seemed plenty strong enough to support their weight. I would still like to try stacking the dipoles on the mast simultaneously in a cross configuration and seeing if that effects anything.
Folded Skeleton Sleeve Dipole
After exploring a few different designs I think that my 75/40 meter Folded Skeleton Sleeve Dipole will make an excellent EMCOMM antenna. It is resonant on both bands, making it easy to tune up and operate on the two most common HF bands used during emergencies. It is also over 20 feet shorter than a typical 80 meter dipole, making it somewhat more space efficient.
While I haven’t done much operating with the Hamsticks, I have done a fair amount of testing with the Folded Skeleton Sleeve Dipole and it is a very competent antenna. My Ham Radio club used it during Field Day and made over 350 contacts with this antenna.