Can I shorten a 80 meter dipole

KF0AWL

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Anyone ever try or know is it possible to take a dipole leg
Say 80m each leg is 68.5.... If I can't fit 68 feet can I take my 68.5 wire run it out half way at 34.25 feed it through a insulator and run it back towards center keeping the wire a total of 68.5 but tech only reaching 34.25?
Hope my thoughts made sense here.
 

jonwienke

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No. That makes a folded dipole at half the length and twice the frequency, and ~300 ohms instead of 50-75 ohms.

If you want to shorten the antenna physically, make loading coils out of the excess wire length by wrapping it around a PVC pipe or similar insulator. Locating the coils at the ends will reduce performance less than putting them in the middle.
 

vagrant

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Do not do that. If you don't have enough room to keep each leg going out, can you turn the legs so they run along your fence line, but not back toward the center? Electrically this keeps the length, but I have no idea what the lobes will look like.

Edit: Or try what jonwienke advised.
 

prcguy

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Sort of but not really. Its possible to fold a dipole element back on itself to some extent and still have the ability to tune for resonance but its efficiency will go down the more you fold back and the closer spaced the folded part is to the original dipole. I don't know how to calculate that without building a model in EZNEC and seeing how it performs.

There is a general rule about shortening a dipole without regard to its impedance in that you can shorten a half wave dipole to about 70% of its resonant length and only loose about 1dB in performance. That would be feeding both the full length and shortened dipole with a high impedance balanced line with no loss and using a tuner at the radio when making the comparison. What your asking is a different twist on this.

Its not uncommon to use a simple loading coil to shorten an antenna but shortening one to half its original length is going to loose a lot of efficiency and also make it very narrow band. If a full size dipole might be useable across maybe 200KHz, one shortened considerably with loading coils might only be useable over about 50KHz.

You could always try it and report the results but it would be nice to have a baseline of a full size 1/2 wave dipole so you know how much it gets screwed up by folding back and shortening it.

Anyone ever try or know is it possible to take a dipole leg
Say 80m each leg is 68.5.... If I can't fit 68 feet can I take my 68.5 wire run it out half way at 34.25 feed it through a insulator and run it back towards center keeping the wire a total of 68.5 but tech only reaching 34.25?
Hope my thoughts made sense here.
 

KF0AWL

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Thanks guys!
🤔 Just a think that crossed my head as I draw up plans for my fan dipole. I have lots of ideas but usually turn to others instead of testing it because someone has usually been there done that lol
Thanks again
 

WB9YBM

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Anyone ever try or know is it possible to take a dipole leg
Say 80m each leg is 68.5.... If I can't fit 68 feet can I take my 68.5 wire run it out half way at 34.25 feed it through a insulator and run it back towards center keeping the wire a total of 68.5 but tech only reaching 34.25?
I've heard with a good enough tuner, you can load up practically anything--if true, I'd say yeah, it should probably maybe work (I've even heard of one op loading up bed springs) although depending on what extremes you go to, you might just end up with a really wonky radiation pattern with no guarantee that the wonkiness is pointed in the right direction...
 

mass-man

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Thanks guys!
🤔 Just a think that crossed my head as I draw up plans for my fan dipole. I have lots of ideas but usually turn to others instead of testing it because someone has usually been there done that lol
Thanks again
Yea, but its the testing that is fun,,,
 

prcguy

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Years ago when I used to check into an 80m morning net, a guy had a 50ft long low dipole fed with 600 ohm ladder line, the really wide stuff. He put in a very respectable signal for such a small antenna being only 25ft each leg where a full size dipole for 80m would be about 65ft each leg. The key here is using a super low loss feedline and 600 ohm open wire is even lower loss than 450 ohm window line. He also had to modify his old Johnson 1kW matchbox balanced tuner to cover the range needed but the system as a whole worked surprisingly well.

If the guy would have used 450 ohm or 300 ohm line it would have degraded some and it would have been impossible to feed directly with coax.

I've heard with a good enough tuner, you can load up practically anything--if true, I'd say yeah, it should probably maybe work (I've even heard of one op loading up bed springs) although depending on what extremes you go to, you might just end up with a really wonky radiation pattern with no guarantee that the wonkiness is pointed in the right direction...
 

mass-man

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I've experimented with HF antennas for years....and as prcguy said, the best so far has been whatever amount of wire I could put up, fed with 600ohm ladder line. You can build your own for not a lot of $$ and it is the most forgiving feedline there is. You have to have a tuner that will handle the balanced line or convert it to coax and then use a short run to the tuner. One of these days I'm gonna get a remote tuner and experiment with that.
 
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