Radiation resistance 1/2 wave vs. 1/4 wave?

[VE3GZB]

Tomorrow with the help of VE3AWA we're going to try to put up a better antenna (probably an inverted V off the old TV antenna tower, Lou is much better at climbing heights than I am).

I have just enough space for a 1/2 wave inverted V on 40m. Not enough space for 1/2 wave on 80m.

But if I wanted to run it on 80m (thus turning it into a 1/4 wave antenna), what would the resonant impedance look like?

73s
geo

[K1JJ]

Geo,

It will be a whole new world for you with a 1/2-wave inv vee on 40M. You will get out like a bomb and start making many contacts.  (I assume it is fed with coax, since you're axing about resonant impedances?)  Make a special effort to get the ends up high and the dipole as flat and straight as possible.

On 75M, the input impedance will be very low and the antenna will be pretty much deaf and dumb. Input impedance maybe about 10-15 ohms.. I'd have to model it at your height to be exact.  The losses will be high and not usable on 75M - if coax fed.  You would need to feed it with open wire to work as a 1/4-wave dipole on 75M or use other tricks.  

There's other ways, but the subject of multiband antennas has been covered in countless threads here so I won't go into it now.  Do a search on this BB and you will find more multi-band ideas using a 40M  1/2-wave dipole.

Have fun on 40M!

Tom, K1JJ

[W2VW]

Coax feed is out unless you have a way to remote a coupler at the feedpoint.

We used a similar antenna at K3L in 2005. Pictures are elsewhere on this site. Radiation resistance AT THE FEEDPOINT will be low as Tom said. With balanced feeders used as a transformer the situation at the shack installed antenna coupler might not be too bad. This depends on feedline length.

With very high SWR and low radiation resistance consider heavy conductor feeders and doublet to cut losses down.

That whole tie the feeders together and work against ground thing is OK only if there is a good ground system to work against. That will yield a low angle antenna and might not work too well for regional work.

[VE3GZB]

Thanks. Yes I will be using 75 Ohm RG/6U to feed it (it's what I have on hand). So I'm better off to abandon 80m on such a small lot and stick with 40m.

I hope I make contacts soon! Right now I'm on dreadfully low power (about 1W) until I get the tubes in to finish the high power Xmitr.

73s
geo
VE3GZB