Is wire length critical in QFH antenna?

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AnomalySmith
Posts: 15
Joined: Sun Jul 10, 2016 1:41 pm

Is wire length critical in QFH antenna?

Post by AnomalySmith » Fri Aug 12, 2016 12:43 pm

Hi!

Almost all is in the title, just wondering if wires length in antenna is really critical and results in more than 5db signal loss ?

I mean with a correctly QFH matched antenna at saying 137MHz but with "extra" coax length at the top (to make it easier to connect each leg) like 5cm at each wire.

Thanks!

joe36
Posts: 36
Joined: Wed Jul 20, 2016 11:09 pm
Location: Cass co. in. 46994

Re: Is wire length critical in QFH antenna?

Post by joe36 » Sat Aug 27, 2016 4:04 am

5 cm will not mater at all. 5 meters at that frequency depending on coax type might make a Db but with a quality coax it would be less than 1. Don’t break the bank for coax. Unless you are going over 50 or more meters. RG6 is a very good choice. Don’t worry about it being 72 ohm. It won’t hurt a thing.
If this is the case where a very long run of coax cannot be avoided. A proper LNA might help. Depending on if the frequency’s you intend to receive are close together. A band pass filter and attenuation after an LNA has many advantages. There are options before going all in with a satellite antennas that is well-tuned quality built and directional. Sometimes this is the only way to get the signal strength you want. Such an antenna setup takes time effort and some $. The dongle is easily overloaded so amplification can often do more harm than good. Most of the time a simple well thought out antenna will work with minimal moneys and a bit of time invested
Joe KA9UCN

JohnM
Posts: 7
Joined: Tue Sep 06, 2016 10:58 pm

Re: Is wire length critical in QFH antenna?

Post by JohnM » Wed Sep 07, 2016 3:50 am

Hi all..

If I understand the question right, adding extra wire to the 4 joins would be like adding 10cm to each length of wire and thus would be tuned to a different frequency.
When using VHF/UHF lengths do become critical... This is not the cable going from the radio to the antenna....its part of the antenna..It would be a good idea to make them per instructions to avoid any issues with them..


John

AD5NL
Posts: 34
Joined: Fri Sep 16, 2016 7:00 pm

Re: Is wire length critical in QFH antenna?

Post by AD5NL » Fri Nov 11, 2016 10:04 pm

If you wanted to be a real purist, you could probably use a quarter-wavelength stub. Although to the best of my knowledge, nobody makes a 61 ohm coax.

You could also try a 12th-wave transformer, which doesn't require funky-non-existent-characteristic-impedances.

https://www.cv.nrao.edu/~demerson/twelfth/twelfth.htm

But again, for receive purposes, 50/52 ohm and 75 ohm coax are basically interchangeable. Don't stress this.

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