135' flattop
 

Hello all ....I have seen some where a list of what length of feedline to
avoid resonance wise....... Does anyone know the site ?? I will be
using 450 twin for the antenna .....never used open wire feed in my 40 years
in this endeavor. Thanks in advance ......


KI3R Tom Popovic Belle Vernon PA ......

135' flattop
 

On Sun, 6 Nov 2005 15:43:14 -0500, "garigue"
wrote:

Hello all ....I have seen some where a list of what length of feedline to
avoid resonance wise....... Does anyone know the site ?? I will be
using 450 twin for the antenna .....never used open wire feed in my 40 years
in this endeavor. Thanks in advance ......

You need only the length of line necessary to make from you rig/tuner
to the antenna. The are some cases that the line length may result
difficulty matching. If so, just add a few feet. Don't worry about
resonance.

73,
Danny, K6MHE

email: k6mheatarrldotnet
http://www.k6mhe.com/

135' flattop
 

Here is a bit I ran across the other day...

I copied this out of the MFJ-269 antenna analyzer manual:
"Resonance at the feedpoint only repeats when a mismatched feedline is
an exact multiple of 1/4 wl. If the line is not an exact multiple of
1/4 wl, the resonant frequency of the antenna might be shifted higher
or lower by the transmission line. A mismatched line that is not an
exact multiple of a quarter-wavelength adds reactance that can either
cancel antenna reactance at frequencies where the antenna is not
resonant, or add reactance at frequencies where the antenna is
resonant."

My deduction:

In essence, if the feedline is matched there is no problem. If it is a
multiple of 1/4 wave in length there is still no problem! Trying to do
both is a good idea.

John Ferrell, W8CCW

On Sun, 06 Nov 2005 13:07:14 -0800, Dan Richardson wrote:

On Sun, 6 Nov 2005 15:43:14 -0500, "garigue"
wrote:

Hello all ....I have seen some where a list of what length of feedline to
avoid resonance wise....... Does anyone know the site ?? I will be
using 450 twin for the antenna .....never used open wire feed in my 40 years
in this endeavor. Thanks in advance ......

You need only the length of line necessary to make from you rig/tuner
to the antenna. The are some cases that the line length may result
difficulty matching. If so, just add a few feet. Don't worry about
resonance.

73,
Danny, K6MHE

email: k6mheatarrldotnet
http://www.k6mhe.com/

135' flattop
 

John Ferrell wrote:

Here is a bit I ran across the other day...

I copied this out of the MFJ-269 antenna analyzer manual:
"Resonance at the feedpoint only repeats when a mismatched feedline is
an exact multiple of 1/4 wl. If the line is not an exact multiple of
1/4 wl, the resonant frequency of the antenna might be shifted higher
or lower by the transmission line. A mismatched line that is not an
exact multiple of a quarter-wavelength adds reactance that can either
cancel antenna reactance at frequencies where the antenna is not
resonant, or add reactance at frequencies where the antenna is
resonant."

My deduction:

In essence, if the feedline is matched there is no problem. If it is a
multiple of 1/4 wave in length there is still no problem! Trying to do
both is a good idea.

Your deduction is not 100% valid. 1/4WL can be a BIG problem and
worst case for matching.

I guess MFJ is technically correct but confusing. For instance,
assume a 50 ohm antenna and 1/4WL of 600 ohm open-wire line. The
"resonant" impedance seen looking into the feedline would be 7200
ohms, outside the matching range for a lot of MFJ tuners. Half a
century ago, we called that the "anti-resonant" point (akin to a
parallel resonant impedance). Most amateur radio operators should
substitute 1/2WL for 1/4WL in the above MFJ manual quote to avoid
thinking 1/4WL is "no problem".
--
73, Cecil http://www.qsl.net/w5dxp

135' flattop
 

That makes sense to me!

Thanks for the correction.
A bold but incorrect position frequently provokes some one who knows
to reveal the correct answers...

On Mon, 07 Nov 2005 15:33:54 GMT, Cecil Moore wrote:

John Ferrell W8CCW

Your deduction is not 100% valid. 1/4WL can be a BIG problem and
worst case for matching.

I guess MFJ is technically correct but confusing. For instance,
assume a 50 ohm antenna and 1/4WL of 600 ohm open-wire line. The
"resonant" impedance seen looking into the feedline would be 7200
ohms, outside the matching range for a lot of MFJ tuners. Half a
century ago, we called that the "anti-resonant" point (akin to a
parallel resonant impedance). Most amateur radio operators should
substitute 1/2WL for 1/4WL in the above MFJ manual quote to avoid
thinking 1/4WL is "no problem".

135' flattop
 

I copied this out of the MFJ-269 antenna analyzer manual:
"Resonance at the feedpoint only repeats when a mismatched feedline is
....[snip]....

Unless I missed it, thta quote from the MFJ manual didn't actually say
"The subject is a single-frequency (or even single-band) antenna". For
a multi-frequency/band antenna, isn't ANY feedline ALWAYS going to be
mis-matched on at least SOME of those frequencies/bands?
--
--Myron A. Calhoun.
Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge
PhD EE (retired). "Barbershop" tenor. CDL(PTXS). W0PBV. (785) 539-4448
NRA Life Member and Certified Instructor (Home Firearm Safety, Rifle, Pistol)