Author Topic: Experimenting with a DIY VHF Dipole  (Read 2721 times)

Offline Greekman

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Experimenting with a DIY VHF Dipole
« on: August 23, 2017, 10:31:16 AM »
So.... Assuming that a dipole has an impedance of 75ohm, which turns to 50 when it is made into an inverted V,

why shouldn't one make an inverted-V VHF antenna and then turn it 90 degrees for vertical polarization?





measurements



« Last Edit: August 23, 2017, 10:36:39 AM by Greekman »

Offline Smurf Hunter

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Re: Experimenting with a DIY VHF Dipole
« Reply #1 on: August 23, 2017, 10:46:52 AM »
Theoretically you should have a 1.5:1 balun between the dipole and coax. 

Also, I think as you get to higher frequencies, your dipole will be less broad banded, and its going to be difficult to get a low SWR across the entire 2 meter band.

We don't usually care about this on HF, both because of lower frequencies and we often have an auto tuner in line.

Offline Greekman

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Re: Experimenting with a DIY VHF Dipole
« Reply #2 on: August 23, 2017, 12:59:40 PM »
yes, i know.
but the goal is to improve on the dipole SWR which is expected to be 1.5. Workable but it doesn't hurt to waste less power

Offline Carl

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Re: Experimenting with a DIY VHF Dipole
« Reply #3 on: August 23, 2017, 05:21:26 PM »
add a couple more elements to the shield connected side and you have a ground plane....

Offline Smurf Hunter

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Re: Experimenting with a DIY VHF Dipole
« Reply #4 on: August 23, 2017, 09:02:18 PM »
add a couple more elements to the shield connected side and you have a ground plane....

If you have a half wave dipole, why should you also need the ground plane?

Offline Greekman

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Re: Experimenting with a DIY VHF Dipole
« Reply #5 on: August 24, 2017, 01:37:12 AM »
add a couple more elements to the shield connected side and you have a ground plane....

in fact i almost did. it so happened i watched a Kevin Loughin's (KB9RLW) suggesting the thing, the day before.

photobucket is acting up, so I will not be posting the pic yet

Offline Carl

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Re: Experimenting with a DIY VHF Dipole
« Reply #6 on: August 24, 2017, 06:04:01 AM »
If you have a half wave dipole, why should you also need the ground plane?

I guess my humor was wasted. You really can't improve the SWR of a dipole without inducing LOSS to the antenna or feedline...1.5 to one is what you get and PLEASE PEOPLE ....remember that SWR is not an indicator of antenna performance any more than your autos 'oil' light is a gauge of RPM, SWR is a measure of the impedance match ONLY and a dipole is a 70 ohm antenna ,so the best you can get ,without inducing loss,is 1.5 to one...or we would all be using dummy loads for antennas.

Offline Smurf Hunter

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Re: Experimenting with a DIY VHF Dipole
« Reply #7 on: August 24, 2017, 07:13:06 AM »
But I can spend a lot of money on a dummy load that gives a perfect 50 ohms any every band.

Offline Carl

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Re: Experimenting with a DIY VHF Dipole
« Reply #8 on: August 24, 2017, 07:28:26 AM »
 :tinfoily:
But I can spend a lot of money on a dummy load that gives a perfect 50 ohms any every band.

Yes ,you can.

Offline Greekman

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Re: Experimenting with a DIY VHF Dipole
« Reply #9 on: August 25, 2017, 05:02:02 AM »
1.5 to one is what you get and PLEASE PEOPLE ....remember that SWR is not an indicator of antenna performance any more than your autos 'oil' light is a gauge of RPM, SWR is a measure of the impedance match ONLY and a dipole is a 70 ohm antenna ,so the best you can get ,without inducing loss,is 1.5 to one...or we would all be using dummy loads for antennas.

well my experiment was to find out if the impedance changes to a much favorable one

Quote
You really can't improve the SWR of a dipole without inducing LOSS to the antenna or feedline
Assuming that -as in my case- nothing changes other than the angle of the elements, where the extra loss would come from?
I mean is there any other kind of energy expenditure other than return and radiated power?