What makes an effective antenna for EMCOMM work?
(HF Antennas the WORK!)
Scott Duckworth, NA4IT
First
of all, who do you need to talk to? Most likely, you need to talk in
your state, or at most two states away. What do we know from our home
HF stations that work for “local” communications?
The dipole.
Usually installed no higher than 40 feet, it works well for local use,
out to say 500 miles. Get it higher, it works farther. Get it lower, it
works closer.
The vertical. This antenna is used by those
desiring to work long distance. Usually, when used to try to talk to
locals, the signals will be well below the signal of the dipole. Why?
Simply put, your signal is going over the station you want to talk to
locally.
So, with these two antennas, which one works the best?
The dipole. NOT the vertical. So, what are we to do when working EMCOMM
(or camping, etc)? We need to figure out how to get a dipole to work on
multiple bands (the ones used for EMCOMM) and be easy to put up.
What bands do we use for EMCOMM? Easy… 75/80M for in state, 40M and 20M for out of state.
Multiple EMCOMM groups (SouthBEARS, SATERN, MARS) use the frequencies and they are listed in their communications plans.
They have used them, and they work.
So, we need an antenna that will work 75/80M, 40M, and 20M. What could we use?
1) A “fan” dipole with multiple legs for 75M, 40M, and 20M
2) A 40M / 75M trap dipole fed with coax
3) HamStick type dipoles.
Advantages / disadvantages for each:
1) A “fan” dipole with multiple legs. A little harder to put up, and takes more room.
Can be built in a “spider” configuration, or with the legs one under the other, longest on top, shortest on bottom.
Feed with coax with a choke coil at the antenna.
2)
A 40M / 75M trap dipole fed with coax. Easy to put up. Raise the center
25 – 40 feet, the ends 6 – 10 feet, connect coax and operate.
Don't forget to use a coax choke at the antenna to stop stray RF from coming down the coax.
3) HamStick type dipoles. If you have access to a portable mast, this antenna can be easiest to put up.
Does require changing antennas to change bands. Not as efficient as full size dipoles.
The ideal antenna would be the fan dipole, with legs for 75M (66.5 feet each), 40M (33 feet each), and 20M (16.5 feet each).
It's entirely possible that a tuner may in fact tune a 75M dipole on 40M and 20M. It's entirely possible that it WON'T!
The next antenna would be the 40M / 75M trap dipole, which has shown it will normally tune the three bands.
It is normally about 55 feet for each leg, for a total of 110 feet in length.
The least effective, but most compact antenna would be the HamStick dipole.
Since each HamStick is roughly 8 feet in length, the total length would be 16 feet.
Again, each antenna is a HORIZONTALY polarized antenna, not vertical.
Each does require some type of mast or center support, and some way to support the ends.
A consideration for radios with automatic antenna tuners: These tuners will normally only tune an SWR of 3:1 to 1 or lower.
This means the antenna you use must be close to resonant to the frequency you are using.
If you need to tune a wider range, consider autotuners like the LDG, SGC, or a good manual tuner.
Why only these three antennas?
Because, they are tried and true. They work. EMCOMM time is NOT time for experiments.
It IS time to put up something that WORKS.
These 3 antennas have been around for years.
There are a lot of “snake oil” salesmen out there.
There are antennas with “matching transformers” that claim to be “all band” antennas.
Just because the radio sees a perfect SWR doesn't mean the antenna radiates effectively.
Some of these “snake oil” antennas radiate a little better than a dummy load.
Diagrams of each of the above antennas follow.
A “fan” dipole with multiple legs for 75M, 40M, and 20M, fed with coax
Trim legs for close SWR match where you operate.
DON'T FORGET the coax choke coil at the antenna!
A 40M / 75M trap dipole fed with coax
Plans for coaxial traps can be found on the internet, or the traps can be purchased pre-made.
DON'T FORGET the coax choke coil at the antenna!
HamStick dipole, fed with coax
Change antennas as needed for each band.
DON'T FORGET the coax choke coil at the antenna!