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Author Topic: Baluns for VHF  (Read 271 times)

KI7LFD

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Baluns for VHF
« on: April 27, 2022, 02:45:22 PM »

Do baluns help improve the performance of VHF/UHF transceivers?

Are there any VHF/UHF antennas that are NOT 50 ohms impedance ?

Thanks in advance for the replies

Regards,
Rich  KI7LFD
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WB6BYU

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    • Practical Antennas
Re: Baluns for VHF
« Reply #1 on: April 27, 2022, 03:20:26 PM »

Quote from: KI7LFD

Do baluns help improve the performance of VHF/UHF transceivers?




They don’t necessarily improve the transceiver, but they
can improve the performance of a station when the antenna
isn’t properly balanced.



Quote

Are there any VHF/UHF antennas that are NOT 50 ohms impedance ?




Lots of them...  depending on what you mean by your question.

Many yagi designs have impedances between about 8 and 30
ohms, for example.  But some sort of matching is used between
the antenna and the feedline to give a 50 (or 75) ohm match.
Whether you think of that matching as part of the antenna or
the feed system would change the answer to your question,
but it is a distinction without a difference.

Because feedline losses tend to be more of an issue at VHF
(and higher frequencies) than at HF, ham antennas are
generally matched at the antenna feedpoint, rather than
using an antenna tuner in the shack.  Some of the older
designs used twinlead or open wire line, and I remember
several tuner designs for 2m and 6m.  But low loss coax
is more commonly available, and much more convenient,
so VHF tuners in the shack are rare these days.

G4AON

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Re: Baluns for VHF
« Reply #2 on: April 30, 2022, 12:16:59 AM »

Do baluns help improve the performance of VHF/UHF transceivers?

Are there any VHF/UHF antennas that are NOT 50 ohms
The modern trend, courtesy of computer antenna modelling, is to add an extra director close to the driven element of a yagi to obtain close to 50 Ohms. 2 element quads can be configured to be 100 Ohms, allowing for a quarter wave matching section of 75 Ohm coax to transform it to 50 Ohms.

Quarter wave sleeves over the coax used to be used as a feedline choke, Tonna antennas used this method. Innov use a ferrite bead choke. Baluns as such are not commonly used, a ferrite bead sleeve choke is a much easier alternative.

In the UK, CPC stock ferrite sleeves that will slide over RG213 size coax, a pack of 5 are worth a try for 2m, two packs for 4m or 6m. A pack of 5 sleeves cost only 1.15 GBP plus tax (around $.1.50 US). Item 559-568 from:
https://cpc.farnell.com/

A simple clamp on RF current meter will show if there is current flowing on the outer of coax, slide it along the cable when putting out a steady carrier. This web site shows how easy it is to make these meters:
http://www.ifwtech.co.uk/g3sek/clamp-on/clamp-on.htm

73 Dave
« Last Edit: April 30, 2022, 12:36:08 AM by G4AON »
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W1VT

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Re: Baluns for VHF
« Reply #3 on: April 30, 2022, 05:40:06 AM »

I believe very large EME arrays have been designed with Yagis designed to be connected together with open wire.

NC1I has a 48 Yagi array for 70 cm.
https://www.qrz.com/db/NC1I

Steve K1FO (SK) came up with the design concepts.
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WA3SKN

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Re: Baluns for VHF
« Reply #4 on: May 01, 2022, 02:11:14 PM »

 Antennas are not 50 ohms.  We make them look like 50 ohms to the transmitter!
The balun is used to keep the coax from radiating a signal and distorting the antenna's pattern.
This may not be important when working local FM/repeater operations, but it becomes VERY important when working with weak signal modes!
So, are you planning a link into the local repeater or setting up for EME?

-Mike.
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