Low Band Antenna

[misterpaul71]

Hi all,

Im throwing this one out there so all the antenna gurus on here can get their teeth into it. Im interested in building something that has a wide bandwidth and covers maybe 33 - 46 mhz. Yeah i know its a big ask.

Up to now i've been using an inverted L made out of wire fed through a 9:1 balun and thats not bad across shortwave and up to about 35 mhz. I've also made a J pole centered on 39.5 mhz which has a bandwidth of about 2 mhz. Dipoles are my other fall back.

Just wondering is there anything else I might try that would work well across this range of frequencies?

Any ideas or suggestions would be very interesting.

Paul.

 

[prcguy]

Its very difficult to make an efficient broad band VHF lo antenna using conventional dipole or ground plane designs. However there are some military antennas and parts from them that show up on Ebay that make great antennas for the 30MHz to as high as 90MHz range.

The military OE-254 is a broad band 30-88MHz Bicone that has three top and three bottom elements about 8ft long and a modified 4:1 balun inside a central hub that secures the elements. These hubs show up on Ebay ocasionally for about $30 on up and make a great starting point to build an antenna. Just add six elements 8ft long to duplicate the original design or make them 10ft long and the antenna will cover about 25 to maybe 70MHz. I use one at home with 10ft elements for 10m and 6m and it works great.

Complete OE-254s with masts, guy ropes, coax and tools show up occasionally but they usually cost more than their worth.

Using the same modified 4:1 balun idea you can also make a "Bow Tie" antenna with similar frequency range and performance. The modified balun is simply adding maybe 10-50pf of capacitance on the radio side of the balun to achieve the best match with the appropriate sized Bicone or Bow Tie. Check this link for some info on the OE-254 and there is a diagram for a field expedient Bow Tie version: Radio communications, light infantry and MOUT

There is also the replacement antenna for the OE-254 called the COM201B which looks like a very fat coaxial dipole with foldable telescoping ground plane elements that double as a tripod for ground mounting. These cover 30-88MHz with slightly better performance than the OE-254 in a smaller package but are usually very scarce and expensive on the surplus market. I just scored two of these antennas on Ebay slightly incomplete but another seller had the missing part for one so I have working unit to experiment with now.

There are also several broad band mobile military antennas than cover 30-90Mhz and if you can supply enough ground plane under them you can get decent performance or at least more consistent performance over a very broad frequency range than a simple ground plane or dipole. Look for the AS-3900 and other variants on Ebay.
prcguy

 

[jhooten]

Dummy load.
Discone
Log periodic
OE-254

 

[mancow]

Well **** now that I googled that I recall seeing it on ebay but didn't know what it was.

 

[prcguy]

I ran an old Hy-Gain 5/8 Penetrator 500 on 10m and a commercial half wave end fed on 6m which were both replaced by an OE-254. The Penetrator was slightly better on 10m but I find no difference between the half wave end fed on 6m and the OE-254. This is using 10ft elements on the OE-254 which lowers the useable freq range some.

I do lots of local 6m simplex FM and SSB and am very familiar with my coverage range before and after the antenna swap and the OE-254 is no dummy load.
prcguy

Dummy load.
Discone
Log periodic
OE-254

 

[jhooten]

If you do decide to go with the OE-254 invest in a good surge protector. They collect static electricity as well as the do RF, maybe even better. I had the coax disconnected from mine for the approaching thunder storms. The static discharge across the PL-259 made a loud pop and crackle. After the show I had to replace the connector. It was pitted so bad. I hate to think what would have happened to the radio had it still been connected.

That list was just a few off the top of my head and in no particular order. The dummy load was listed because I once made a DX contact on 6m SSB then looked at the antenna switch to see which antenna I was on and it was switched to the dummy load. Contact was about 2500 miles on 25 watts to a dummy load with S9 signals on both ends. Ain't radio fun?

 

[k9rzz]

The losses you encounter from SWR for receiving purposes are negligible.

I would have one vertical antenna and one horizontal antenna up as high as I could.

That's it. Having more antennas than that will not force the band open to you.

 

[zz0468]

That list was just a few off the top of my head and in no particular order. The dummy load was listed because I once made a DX contact on 6m SSB then looked at the antenna switch to see which antenna I was on and it was switched to the dummy load. Contact was about 2500 miles on 25 watts to a dummy load with S9 signals on both ends. Ain't radio fun?

Yep. it sure can be fun when that sort of thing happens. But I would give both the dummy load and the cables a good hard look. One or more of those items isn't any good.

 

[prcguy]

Do you mean like a VHF low band antenna tuned for one frequency and at another freq within VHF lo band but away from the low SWR spot there will be negligible loss? Not so.

I tested among other antennas, an Austin Spectra that was factory tuned at 33MHz for just this same topic. Using another reference antenna on the same mount I measured the Austin Spectra down 19dB at 40Mhz and right about 40dB down at 50MHz keeping the original 33MHz tuning on the Spectra.

That's with the Austin Spectra receiving known distant stations and not transmitting and the degradation away from the tuned spot was huge.

A full size 1/4 wave ground plane may not have as much degradation as the shortened and loaded Spectra when used away from its tuned frequency but it will absolutely drop noticeably in performance as you get away from the sweet spot.
prcguy

The losses you encounter from SWR for receiving purposes are negligible.

I would have one vertical antenna and one horizontal antenna up as high as I could.

That's it. Having more antennas than that will not force the band open to you.

 

[jhooten]

Yep. it sure can be fun when that sort of thing happens. But I would give both the dummy load and the cables a good hard look. One or more of those items isn't any good.

The great Bastrop County Labor Day Conflagration of 2011 solved all of my station problems. But I was suspecting the switch more than anything else.