Sunday, March 10, 2019

The 43 foot HF vertical antenna

There's a lot of mention (particularly in articles by US hams) of the 43 ft HF vertical antenna. What's so special about it? And is there any difference between it and one that's say 40 ft or 46 ft?

The first thing is to convert its length to metric. I don't know why people still use imperial measurements for antennas when radio wavelengths and amateur bands are expressed in metres. Antenna lengths have a relationship to wavelengths, so using the same units is important to know what's going on.  Then we can work out in our head whether 43 ft is approximately 1/4 wavelength, 1/2 wavelength, 5/8 wavelength or whatever for our band of interest. 

Anyway, 43 feet is near enough to 13 metres. That's roughly 5/8 wavelength on 20 metres (14 MHz).  5/8 wavelength, while not resonant, is a 'sweet spot' length for low angle radiation on the busiest DX band. An antenna with a concentration of low angle radiation is important if you, like most 14 MHz operators, aim to work DX.

On 17m (18 MHz) it's about 3/4 wavelength and on 10 MHz it's a bit under 1/2 wavelength.  40 and 60 metres? A bit over a quarter and a bit under a quarter wavelength respectively. So you should be able to load up something that's 43 ft long on those bands also. 

I would be slightly concerned about 80 metres (3.5 MHz) though. Here it's getting a bit short. Only 0.15 wavelength. But I wouldn't give up. Some loading, eg provided through a capacitive hat and/or a loading coil should do the trick. But expect the bandwidth to narrow so it won't cover all the band, particularly in countries where it's 500 kHz wide such as the United States. Still it's vastly better than nothing, especially if you can't put up a full quarter wavelength vertical (substantially longer at 20 m).

As for 1.8 MHz, or 160 metres, it really is a compromise down there. Expect substantial loss. But again don't give up as, if it's any consolation, most other people on the band are also using compromise antennas.  Some lower than yours. 

13 metres isn't a resonant length on any amateur band. So we need to do some work to make it usable. Yes, you will need an antenna coupler with this antenna. But the trade-off is that it should work on multiple bands. One tall, slim, elegant antenna like this might be more acceptable to others at home than a tangled web of multiple wires for different bands.

If you're a DXer, provided you've done the groundwork regarding radials and aren't throwing away power in compromise matching systems, a 13m vertical could be a good choice. Especially if multiple band capability is important.   

What do others think?  Read their thoughts and construction ideas below: 

PS: Want to read even more about vertical antennas? Consider this selection of antenna books. They are affiliate links meaning that I receive a small commission (at no extra cost to you) if you decide to purchase.

1 comment:

  1. A multiband antenna is always a compromise. I use them and that's what I think. 73, Bas


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