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We're putting up an 80m dipole at the K0KU club station, but need to cover most of (if not all) of 75/80 meters.
My current thoughts on achieving this is a parallel dipole, with the upper wire cut for 3.550 and the lower for 3.900 (which are our target areas). If the (-) is the wire, it'd look something like this:
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What are your thoughts?
Any other recommendations?
Only requirement is that it's coax fed.
Cage antenna - with ~12" diameter elements - or a folded dipole with a 4:1 balun at the feedpoint.Quote:
Originally Posted by KU0DM
How about a fan dipole? Cut one dipole for 3.600 and the other at 3.900, feed both as you would feed a single dipole.
If you have the $$, How about a 5BTV?
Or a reduced footprint dipole? You could add the extra elements as needed to get full coverage.
I would go with the 6BTV. The 5 has capacitance hats on top of the 20 and 40M elements; these add a considerable amount of wind load. The 6, on the other hand, makes up the required length by virtue of its 30M trap and intermediate sections.Quote:
Originally Posted by N2NH
If you have the room (presumably on the building roof?) I'd build two dipoles -- one cut for 3550, one for 3900 -- that share a common feed point. Mount one in one direction (say, NS), the other 90 degrees to the first (say, EW).Quote:
Originally Posted by KU0DM
Energy flows along the path of least resistance, or in the case of antennas reactance, so while there will be some interaction between the two, it won't be much. The effect will be that one antenna will be doing the work when the other (due to high reactance on that particular frequency) will be pretty much inert.
I've done this before, and it works well (both in the 80 CW/75 Phone configuration, and 80 CW/40 CW configuration). You may have to do some minor tweaking of the antennas to get them to play on the frequencies you want, so cut them a touch long and trim accordingly.
I don't know what would happen if you hung the two antennas in parallel with each other, such as this:
80 CW ------------------||------------------------ ____________(similar to a catenary wire used on electric trolleys and rail lines)
75 PH __ --------------- ||--------------------
(ignore the __ inserted to get the spacing right)
The interactions would be different, but I think in general it could work as well.
Either way, you would need to plan on a transmatch to work the DX phone part of the band.
Also, either way, try to raise the center feed as high as practical, to make the antennas more of an "inverted V" than a true "in plane" dipole. Brings the feedpoint impedance closer to 50 ohms than 75, and the better the feedpoint impedance, the easier it is to match the antenna with 50 ohm coax.
Neither vertical is broadbanded enough on 80 meters to give them the range they want. Even with a transmatch, there are still practical limits.Quote:
Originally Posted by N8YX
I love my Butternuts, but even though they (and allegedly the Gaps) are have a wider 2:1 - 2:1 bandwith, that's just too big a stretch.
An end fed 43' vertical would give them the range, but again, a transmatch would probably be required. Frankly, if the club has a tower for the higher bands at it's disposal, I'd consider loading that with a gamma match to work 80/75. And any end-fed vertical would need a decent to good radial field underneath it, but that should go without saying (and more often than not does)
Why?Quote:
Originally Posted by KU0DM
And what's the purpose of this dipole? Local (under 500 miles)? DX (greater than 1,000 miles)?
A vertical of any flavor that isn't full-sized isn't going to have the bandwidth required either, and it's not very good at under 400-500 miles. Verticals shine as DX antennas.
That, or a sloper arrangement with the lower part folded back into the tower and a set of relays to switch in a piece of wire at the ends of each for CW-band coverage.Quote:
Originally Posted by W3WN
With a decent radial setup (elevated mounting) I see a 2:1 SWR B/W of about 75KHz with my 6BTV, and I've parked the "dip" right in the middle of the CW subband. This, by the way, is using the "Super" resonator.
Ended up going with a single dipole for CW.
Purpose: Domestic contesting, and H&P during DX contests.
Why coax fed?: Simplicity. That and lack of ladder line.
Vertical vs. Dipole: Has to be on the roof, that's our only approved play area (Fortunately it's on a 50' tower atop a ~80' building), and I doubt we could get away with another tall structure.
Why coax still ? Ladder line is cheap and can even be hand made. Or, in a pinch 300 Ohm TV Twin lead can be used. Cut the antenna for 80, feed via link coupled tuner...bought or built. For the antenna and 14, 12, 10 gauge wire will do. Old railroad telegraph wire is perfect.Quote:
Originally Posted by KU0DM
If it's DX ur after, go with 4" diameter irrigation pipe for the lower 40', then a 30' length of 3" swaged into the top of the 4". Feed with 50ohm coax direct and it covers the entire 80 meter band, and then some, with less than 1.5 to 1 SWR the whole way. Very broad banded. Currently using 4 of these as an 80 meter 4 square.
Quote:
Originally Posted by Vinnie
Ive seen a 160m four square like that...used irrigation pipe as well!... woof! would love to have that kind of property to put that up!
check this site out to make one yourself
http://www.hamuniverse.com/antennas.html
You're right. The 2:1 SWR bandwidth on a 6BTV on 75/80 meters is very narrow (like 80KHz). I'm liking both the multiple dipole (fan dipole) and the cage ideas presented here. Might even be able to go with three dipoles in a fan configuration to flatten out the center of the band.Quote:
Originally Posted by W3WN
Much beyond that, I'd be tempted to just tune a long wire. That could be fed with coax without any real problems. I haven't done much with long wires on 80, but ran one on 40 a couple years back in a portable situation. Wire couldn't have been 150' long, thrown in some trees and tuned with a Palstar AT1KM. Was working ZL/VK easily with 30-35W on CW. Very low noise too as I recall.
Got it squared away. Someone donated an HF9V with a 160 coil, so we are putting it on the upper roof (about 80' in the air) as an elevated vertical.
Your going to need some sort of counterpoise or radials to make it work proper. Tie into the rain gutters and ground them or some thing.
Three 1/4 radials per band. Being elevated that's all it needs.Quote:
Originally Posted by N9FE
Roger