[Antennas] Coaxial feed line question.

Wed Mar 1 20:23:22 EST 2006

Put the antenna up with any length coax......the 1/2wl thing is more hype
than anything (and may hurt you if you try to use the antenna on the 3rd
harmonic on 15mtrs!)....If your antenna is trimmed to a 50ohm match at its
feedpoint, ANY length of 50ohm coax will show 1:1 SWR at the xmtr....Of
course a REAL dipole is closer to 70-75 ohms than 50..but when it is low to
the ground (less than a wavelength or so), all bets are off in my book...and
you just need to trim it for 1:1 match.
Measure your SWR AT the antenna if you can....but any length of coax is
fine...a choke balun is made of 7-10 turns of coax in a semitight coil (a
oatmeal cynlinder will do as a form) and will work on any band ok...
Chris
WB5ITT
> -----Original Message-----
> From: antennas-bounces at mailman.qth.net 
> [mailto:antennas-bounces at mailman.qth.net] On Behalf Of Sam Morgan
> Sent: Wednesday, March 01, 2006 11:29 AM
> To: antennas-qth.net
> Subject: [Antennas] Coaxial feed line question.
>> I have a question related to the length of a coaxial feedline 
> for a 40m antenna. I want to use a length equal to 1/2 
> wavelength electrically. The velocity factor of the feedline 
> is 66% (RG-213) So using 7.1 mhz. as the frequency, I come up 
> with 45.84 feet. Seems I remember this 'multiples of a
> 1/2 wavelength feedline' as some magical thing that 
> effectivly places the antenna feedpoint in the shack at the 
> end of the feedline??? Been way to many years since I did 
> this stuff, lol. Comments please.
>> I have seen a chart in the handbook that says for 7 mhz a 
> coil of RG-213 of 22 ft in 10 turns will be an effective 
> choke for this setup. Or a coil of 10 ft,
> in 7 turns would work for 3.5-30 mhz.
>> 1st question is, if either coil is added on the end of the 
> basic 1/2 wavelength 45.84 foot feedline...... will this 
> defeat my purpose and change the length of the feedline? Or 
> is the coil some how just a blackbox kind of thing, not 
> effecting the 1/2 wave length calculation?
>> 2nd question:
> If I tried to use the above '40m 1/2 wave feedline and coil' 
> on any other bands, by running it thru a tuner in the 
> shack,.... I blow the entire thing and the feed line 
> radiates... yes/no?
>> --
> GB de ka5oai,
> Sam Morgan
>