Is wire length critical in QFH antenna?

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Is wire length critical in QFH antenna?

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


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.


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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

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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..


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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.

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

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