I wanted to build a vertical antenna for portable use. The antenna should cover all bands from 10 to 20 meters. Light weight and easy handling were my priorities since I plan to use the antenna for backpacking and SOTA activities.
At this time I own only one HF radio – a Yaesu FT-818ND. That’s the tried and trusted solution for us wanna-go-out-and-play-radio-in-the-outback-guys. With my not so loud six watts I need to get as much as possible of my signal out there. I want to run digital modes as well as voice, hence my antenna needs to be fairly broad banded.
For portable operations believe changing bands won’t be a burden of any kind, so I didn’t need to build a true multi band antenna, but rather an antenna with an interchangeable radiating element. I thought about building the antenna as a linked antenna, but eventually came to the conclusion it’ll be just as easy changing the entire element.
To summarize, these were my demands:
- Homebrew – of course!
- Light weight
- Easy to assemble and disassemble
- Cover the entire 10, 12, 15, 17 and 20 meter bands for digi and voice abilities
- Does not need to be a true multi band antenna – interchangeable radiating element is OK
Building the antenna
The classic telescopic fishing rods are inexpensive and fairly light. I had two five meter rods already, and started out with one of them as antenna mast. The easiest approach (and probably most light weight) would be the Marconi style vertical – a single quarter wave vertical radiating element and radials laid on the ground.
I decided to build a “distributor box” that could be placed on the ground at the base of the antenna mast. This distributor would accept the coaxial cable and allow for quickly connecting and disconnecting both the radiating element and the radials. I purchased banana plugs and sockets on Ebay. These turned out to be of surprisingly decent quality. To house the connectors I used a plastic box also bought on Ebay. I had the box already sitting in my drawer awaiting a project like this.
Radiating element and radials
The radiating element should be a quarter of a wavelength. I dimensioned for center of the bands since I wanted the ability to run digital modes and voice. On 20 meters the center frequency is 14.175 MHz (here in Norway). A quarter wavelength is 5.29 meters. The velocity factor in solid copper is approximately 0.95-0.97. I figured I would rather cut the wire a bit too long and cut it to resonance afterwards. I cut it to approximately 5.6 meters (I didn’t measure exactly).
The length of the radials is not critical when they lay on the ground as in the case of my antenna. I read somewhere that radials longer than 0.28 times the wavelength is not necessary. More gain is achieved with more radials instead. I also read somewhere that 16 radials is a good number. The difference between 16 and 32 radials is said to be marginal. I cut my radials to 1.5 meters, and made four groups of five radials – twenty in total.
I equipped each group of radials with a banana plug which would go into the distributor. I filed off the enamel, twisted and soldered the ends together, tightened the screw in the plug and soldered the entire thing to ensure good connection. The plugs were easy to solder as long as I used enough heat.
The radiating element was terminated the same way as the radials, only with just one wire. I made just the 20 meter element to start off. The rest of the elements can wait until initial testing is done.
With a 5.6 meter radiating element my fishing rod was a bit too short. I lengthened it with a piece of PVC pipe I had lying around. The copper wire extended approximately 20 cm above the top.
I was surprised to find the SWR meter on my FT-818ND showed zero on the entire band on first try. I intentionally cut the wire longer than I calculated, but it seems I hit spot on. The internal SWR meter is known for not being accurate, but in lack of a suitable SWR meter or antenna analyser I have no means of adjusting it further at this time.
When I had hooked up the antenna I had a few contacts on FT8. The propagation was not the best according to the charts, but for the first time on 20 meters I received better reports than I sent (usually I use an 84 meter sky wire loop antenna). My conclusion is that this antenna is successful!
It was surprising to find the SWR not even tickeling the meter on the FT-818ND. In time I probably will ask a fellow ham to borrow his analyser to adjust the radiating element properly. But I will wait until I have made all elements so I can adjust all of them at the same time.
I will have to buy a six meter version of the fishing rod. The PVC pipe solution works well for initial testing, but it’s not very sturdy and not usable in the long term. The six meter version is long enough and not expensive.
How will it perform in the winter?
I wonder how this antenna will perform on ice and snow. I live in a part of the world where winter is cold and snowy, and I am curious to see how the antenna will perform when it gets deployed on a layer of snow or ice instead of directly on the ground. I believe this is similar to elevated antennas where the radials have to be resonant. Time will show – the very first snowfall is expected within a couple of months…
A big thank you to DK7ZT, SP7TF and MW3FLI for the contact tonight!73 de LB1RI