Using the dipole antenna kit - setting correct lengths

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Davespice
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Joined: Fri Jun 14, 2019 9:53 am
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Using the dipole antenna kit - setting correct lengths

Post by Davespice » Mon Sep 07, 2020 2:25 pm

Hi folks

I need some help getting my head around something, is there anyone here who's familiar with the RTL-SDR dipole antenna kit product? The one described on this page: https://www.rtl-sdr.com/using-our-new-d ... tenna-kit/

I've been using it, fairly successfully, for about a year now. I recently tried to use it with the short telescopic sections to try and receive the downlink of the cross-band repeater on board the ISS. Which is on 437.8 MHz.

In terms of calculating the lengths of the telescopic sections the article I linked above has a section called Choosing the Antenna Element Length. It advises to use this dipole calculator website to calculate the length that will make the antenna resonant frequency match the frequency you are interested in. There's also a little cheat sheet table for various common frequencies.

Now, I can only make the dipole calculator website match the cheat sheet if I select One side. To me it makes sense to do it this way because you have two telescopic sections to set and therefore you would probably do them separately.

However I was recently in a discussion with a colleague who was advising I should set the calculator to Both sides because this will give a value which is closer to half the wavelength at the desired frequency. Now this is where I am getting confused.

If it is a half-wave dipole then the full length of the dipole would be half of the wavelength right? So each side is a quarter of the wavelength.

Or is it a full-wave dipole and each side is half of the wavelength and the full length of the dipole is equal to the wavelength?

Which is it?

Sorry I know this is probably novice level stuff, but by Google Foo seems to be failing me.
Many thanks in advance

Dave 2E0XDO

rjmura
Posts: 14
Joined: Sun Jun 02, 2019 3:57 am

Re: Using the dipole antenna kit - setting correct lengths

Post by rjmura » Mon Sep 07, 2020 9:30 pm

the total length of a half wave dipole should be a half wavelength. so for 437.8 MHz, the total length (both sides) is 32.7 cm, and from center to one end is 16.3 cm. the short antenna fully extended has a total length of 32 cm, which should be close enough.

the attached vna plot of the short antenna fully extended shows the resonance at 441 MHz and a VSWR of 1.148.
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