80m Dipole

[KI4SYC]

I have the room to erect an 80m dipole in my backyard. However, it seems some information isn't the same. Is there an exact length for an 80m dipole? I've also read it will work on all bands. I don't know if that's true, but perhaps it might with some compromises.

[WJ5O]

Ira,
A 80 meter dipole with 120 foot legs fed via ladder line and a good tuner will work all the HF bands except 160 M

[KI4SYC]

Thanks for the reply. I thought it was in that neighborhood. However, I have seen different lengths. So no luck on 160m with it? Oh well, you can't always get what you want.

[KA0GKT]

Quote[/b] (KI4SYC @ May 02 2007,18:44)]Thanks for the reply. I thought it was in that neighborhood. However, I have seen different lengths. So no luck on 160m with it? Oh well, you can't always get what you want.

Actually, the antenna can be used for 160, just not as a flat-top...and you'll need a set of radials.

What you do is tie both sides of the ladder-line together and feed the transmission line as a vertical over the set of radials with the 80m flat-top a big capacative hat.

Is the antenna efficient when operated on 160 in this manner? No, but it does radiate and get you onto top band.

73 DE KAØGKT/7

--Steve

[N5KRC]

Quote[/b] (KI4SYC @ May 02 2007,12:25)]Is there an exact length for an 80m dipole?

Of course there is, but it's going to vary from one install to the next. Height above ground, nearby objects, your soil, etc. will all play a roll in it.

468/f is the formula for calculating a half-wave dipole. Pick the frequency you want it to be resonant on, and then use the formula. Take the number you get, and add a few feet to it. Once your dipole is up, you can then trim the ends and find that 1:1 match on your particular frequency.

Quote[/b] ] I've also read it will work on all bands. I don't know if that's true, but perhaps it might with some compromises.

It will, provided you feed it with ladder line and use an external tuner. This is known as a doublet antenna. The only compromise is it's harder to run ladder line than coax. In fact, if you put one up that's resonant on eighty meters, and use it on the lower bands (higher frequencies) with your tuner, it will actually increase your gain on those bands.

[W5DXP]

Quote[/b] (N5KRC @ May 03 2007,08:10)]468/f is the formula for calculating a quarter-wave dipole.

That's an obvious typo. You no doubt meant to say, "468/f is the formula for calculating a half-wavelength dipole."

[N5KRC]

Quote[/b] (w5dxp @ May 03 2007,01:25)]

Quote[/b] (N5KRC @ May 03 2007,08:10)]468/f is the formula for calculating a quarter-wave dipole.

That's an obvious typo. You no doubt meant to say, "468/f is the formula for calculating a half-wavelength dipole."

That's exactly what I meant to say... One of these day's I'll learn to refrain from posting until I'm on at least my second cup of go-go juice! Thanks for catching that, don't want to pollute the already murky waters in his head.

[WA2ZDY]

KRC is right about using it as a multiband antenna though. # If you decide to do that, whatever else you do, DON'T feed it with coax. #You need the external tuner, a good 1:1 current balun and ladder line from the balun to the antenna.

You can remotely locate and auto tuner and run coax from the rig to the tuner - the tuner keeps the SWR flat at the end of the coax. # And most autotuners are only for single wire output for random wire and bottom fed whips. #Here's how I connected the balun to mine:



The braid of the coax is screwed down under one of the cabinet screws. Be careul by the way. If you google this idea, there is a ham with a website showing photos of his setup that are not the way to do this! He shows twinlead connected with one side to the hot terminal of his AH4 tuner and the other to ground. That defeats the whole point of using balanced feedline. (I emailed him about it and got no reply.)

Any center fed antenna can be used as a multiband efficiently on any band where the antenna is at least a half wavelength long and it's fed in this manner with balanced feedline. #That's why the instructions for using it on 160 are different.

Hope you don't mind this extra info. #I know you only asked about 80M.

[W5DXP]

Quote[/b] (WA2ZDY @ May 03 2007,08:46)]Here's how I connected the balun to mine:

There is a problem associated with that configuration. If the balun opens up or the coax breaks, the SGC autotuner is capable of supplying more voltage than the coax can tolerate resulting in arcing. If it occurs in an attic, such arcing could possibly burn down a house. SGC recommends against that configuration.

[K3WRV]

The formulae given above are correct 468/f for a dipole 1/2 WAVELENGTH HIGH (that would be about 67 feet at 3.725 Kc Just use an swr meter and if the resonant point is below where you want, make it shorter. And if it's higher, make it longer. (??? or it might be the other way around - lower ht = lower impedence&#33

The formula is 492-n/freq to allow for end effect (abt 5%), but most of us make them cut and try, or use open wire (or good quality tv Twinlead and a tuner). The ANT WILL NOT work on all bands with Coax!!! That only works with ladderline (or twinlead)

BTW, Twinlead will work up to about 300 W.

gl es 73 de bob