Author Topic: feedline length question  (Read 204 times)

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GreekMan

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feedline length question
« on: May 02, 2019, 01:48:14 AM »
hi all! missed you some, cos I was away for the last 8 months...

so i come back with a question.
How critical is coax feedline length when it comes to antenna matching? (in both HF and VHF?)
I once saw that your line should be in multiples of the wavelength.
Does this hold true?

ghrit

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Re: feedline length question
« Reply #1 on: May 02, 2019, 10:00:17 AM »
Welcome home.  Did they feed you well enough?

IIRC, the general rule is that feedlines should NOT be multiples of half lambda regardless of band.  With care, you can pick a length that is not resonant on antennas on any wavelength.
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AD

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Re: feedline length question
« Reply #2 on: May 02, 2019, 10:21:16 AM »
I need to back and read up on this for my gear.
The only dumb question is the one that did not get asked!!

BTPost

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Re: feedline length question
« Reply #3 on: May 03, 2019, 12:35:01 PM »
In practical reality, feedline length is irrelevant to anything, if both Source and Load are at the same Impedance, AND of course the Line Loss of the Feedline itself, is relevant.... Where you get into problems is when that is NOT the case... If your Antenna is NOT the same Impedance as the Source, (i.e. Frequency is NOT at Resonance point of the Antenna) then things can get a little wonky... The worse the Impedance mismatch the more wonky things get... This is why Autotuners, at the Source, work well... They help match the source Impedance to whatever the Load Impedance turns out to be at the frequency of the RF... At LF/MF/HF these difference can get out of hand very fast as the Antenna Impedances change rapidly as a percentage of Resonance Mismatch.... At VHF/UHF you have to change the Frequency a LOT to get a significant difference in Antenna Impedance... (i.e. USUALLY YOU CAN GET A WHOLE BAND TO BE GOOD, FOR A GIVEN ANTENNA, WHERE AS AT LOWER FREQUENCIES, SOMETIMES YOU ARE LUCKY TO GET EVEN 1/3 OF THEN BAND TO WORK WELL)
Bruce in alaska AL7AQ

GreekMan

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Re: feedline length question
« Reply #4 on: May 08, 2019, 05:26:42 AM »
thanks for the answers guys

(sorry for the late reply but I had to let info to sink in)