Around a year ago I started getting back into backpacking, calling up and organizing trips with some old friends. I knew from the start that I wanted ham radio to be a part of these adventures. Along the way I've been refining the setup that I've taken along.
Antenna Purpose
Reducing weight is super important on the trail, which has led me to try to be very pragmatic with the equipment I bring along. Let's face it, radios are (generally) heavy. When I turned my attention to review the antenna I was bringing I started by trying to figure out specifically what I wanted the antenna to do:
The antenna should enable communication from a few tens of miles/km, to about 500mi/800km at almost any time of day.
The reality is that I want this antenna on the trail so I can contact friends and family while I'm out, and as a way to contact help should I get into any trouble out on the trail.
Band Selection
Even though I had a hunch on which bands to choose, I wanted to rieview some data and let that guide my decisions. I went over to voacap.com/hf, which is a wonderful tool to visualize many things about radio propagation. I changed the settings to a 5W CW station and made Reliability, "REL", plots for each band. Looking through the plots it became clear that three bands in particular stood out for this application: 40m, 30m and 20m.
The 40m band, 7MHz, is a wonderful band. During the day when the ionosphere is energized by sunlight, signals from this band are refracted/bent more, shortening the skip and sending the energy locally. The skip is shortest when the sun is roughly highest in the sky at local noon. Then as the sun sets the ionosphere is no longer directly illuminated causing less refraction, which lengthens the skip propagation at night. Near midnight there may be weakening or loss of some very local stations.
40m:
As we move higher to the 30m band, 10MHz, we see clearly that higher frequencies are refracted less leading to further skip distances. During the day we see similar good local propagation, but also more distant propagation is enhanced. At night the skip distance increases and local propagation becomes unreliable.
30m:
And finally to 20m, 14MHz. The highest in frequency of the three this band displays great daytime propagation out to around 1000km. Sometimes signals on this band aren't refracted enough for local communication making this better for longer contacts during the day. At night as the refraction lessens and this band "goes long" or possibly doesn't support contacts at all.
20m:
If we consider all three bands together we see that areas out to about 500mi/800km are predicted to be in reliable propagation on one of the three bands, at almost any time.
There is a short time for about two to three hours before sunrise where propagation from 0-250 miles is predicted to be unreliable. The effect of this is that while I would probably be able to complete a contact to someone in an emergency during that time, I'll probably be out of contact from friends and family because I'll usually be less than 250 miles from them. Honestly, I hope I'm asleep, or waking up for a morning hike to the sunrise at those times. In any case I decided that this was an acceptable compromise/consequence for the design, so I continued on and looked to what type of antenna to make.
Antenna Design
On my first outing I initially experimented with a 20m vertical, with a tunable loading coil. This worked okay but it had some problems:
- It was heavy, not good for backpacking.
- Changing bands took antenna adjustment and verification (time+energy)
- It required many radials.
- Reduced efficiency on bands below 20m as the antenna became small compared to wavelength.
- It required a tuner to improve the match on most bands.
So I looked around and started playing with end-fed half wave designs. End fed designs are convenient for backpacking and portable operations. When the end of the antenna is placed near the operating position (like your tent) very little coax is required which means less weight! They can also be setup like dipoles or verticals and do not require ground radials.
The idea of getting setup in a tent, and then needing to exit or adjust every time I wanted to change bands didn't sound appealing so I decided against a linked EFHW. I started thinking about adding traps so no physical adjustment was needed between bands. This method is also an appealing way to make remote digital stations multi-band, as it doesn't require any interface to a tuner.
I decided to use traps to produce 3 resonances, on each of the bands chosen above. Traps stop energy from flowing past them by acting like an open circuit. If we tune these and position them carefully we can produce multiple resonances at the bands we want.
Signals at 7MHz energize the entire antenna, passing through both traps. Signals at 10MHz flow past the first trap designed for 14MHz, but can't pass the second 10MHz trap (because the trap appears like an open circuit with very high impedance at this frequency). Similarly when transmitting on 14MHz energy stops at the first 14MHz trap, making the antenna appear even shorter at this frequency.
Using traps does shorten the antenna a little but not by much. This antenna has the advantage of being full sized on each band . The traps, when made well, should not contribute much to the loss within the antenna. I decided to use air core inductors and silver mica capacitors to keep trap losses low.
After prototyping the antenna in my back yard over a few days I had an antenna with a good match (SWR of about 1.2 or 1.3 to 1) on 40m, 30m, and 20m. One thing I've learned though in engineering is that a prototype is just the start. A prototype can work well in a controlled environment, but it really needed tested doing the things it was made for: backpacking and POTA.
The final antenna came to this design:
Setup and Operation of the 40/30/20 EFHW
My normal setup routine for the antenna:
- Find an acceptable place to sit, within about 20-50 feet of a tree/support.
- Prepare the throw: Prep mason line and arborist weight.
- Choose a support 40-50ft high and send the arborist weight over it.
- Secure both ends of the antenna with mason line. Raise the antenna.
After a little practice, I can lazily set this antenna up in about 10 minutes.
On my first few outings with the antenna I would check the match on the nanoVNA I have. I learned two guidelines to avoid ground losses:
- The antenna feed point should be at least 3ft/1m above ground, especially if the antenna is at all horizontal.
- If the entire antenna is low it is best at 6ft/2m or more above ground.
When the antenna is lower than this, ground interactions cause the match to worsen, reducing the efficiency of the antenna. Even if the antenna is setup low though, it will still likely produce easy contacts. I once forgot the mason line I use during POTA activations. I got around this by attaching the arborist weight with a small piece of string I found and threw it over a low hanging 10-15ft/3-4m branch. I was surprised how well the antenna performed and how quickly I was able to activate the park even with a low hung antenna. Height helps though. If you can't hear them you cant work them. In general you'll have more success by getting the antenna higher off the ground.
POTA with the 40/30/20 EFHW
I love this antenna for POTA. After set up I usually start out on 40, and move up to 30 then 20 if the contacts become slow. Changing bands is effortless. I didn't fully realize how nice that was until I just didn't need to think about it, after coming from manual tuners. It is especially nice if you're tuning around and hunting stations! I chose orange for the wire color of my POTA antenna, and it is easily visible for added safety. Green blends in well too, if you're not looking to stand out.
A short video I took of deploying and using this antenna for a POTA is on YouTube at: https://youtu.be/4mi0RqhVXIM
See you down the log!
K4SGP - Jim