Instead of wasting bandwidth on stuff that cannot be changed by the general
populace (CW requirements) or stuff that is just to jerk peoples collective
chains (E-Bay posting in this NG), how about something that might even be
technical in nature??? How about the answer to this question??.....When
cutting the feedline for a dual band (2m/70cm), for maximum performance, it
needs to be cut to multiples of electrical half wave length. Does it need to
be cut for 2m? or 70cm? a combination of both? or split the difference?


--
Steve @ Noon-Air Heating and A/C


When a work lifts your spirits and inspires bold and noble thoughts in you,
do not look for any other standard to judge by:
the work is good, the product of a master craftsman.
- -- La Bruyere

Noon-Air wrote:

Instead of wasting bandwidth on stuff that cannot be changed by the general
populace (CW requirements) or stuff that is just to jerk peoples collective
chains (E-Bay posting in this NG), how about something that might even be
technical in nature??? How about the answer to this question??.....When
cutting the feedline for a dual band (2m/70cm), for maximum performance, it
needs to be cut to multiples of electrical half wave length. Does it need to
be cut for 2m? or 70cm? a combination of both? or split the difference?

Don't think it's really necessary to cut to any particular length at all.



--
Steve @ Noon-Air Heating and A/C


When a work lifts your spirits and inspires bold and noble thoughts in you,
do not look for any other standard to judge by:
the work is good, the product of a master craftsman.
- -- La Bruyere

When
cutting the feedline for a dual band (2m/70cm), for maximum performance, it
needs to be cut to multiples of electrical half wave length. Does it need to
be cut for 2m? or 70cm? a combination of both? or split the difference?

As an old friend of mine (yup, I've got friends...) once said, "Make the coax
feedline just long enough to reach the rig."

Fits most (but not all) situations.

A
"...if one tolerates bad manners, they grow worse. I must find the oaf who did
this thing, explain to him his offense, give him a chance to apologise, then
kill him." (R.A. Heinlein, "The Cat Who Walks Through Walls")

"AA" wrote in message
...
When
cutting the feedline for a dual band (2m/70cm), for maximum performance,
it
needs to be cut to multiples of electrical half wave length. Does it
need to
be cut for 2m? or 70cm? a combination of both? or split the difference?


As an old friend of mine (yup, I've got friends...) once said, "Make the
coax
feedline just long enough to reach the rig."

Fits most (but not all) situations.

It seems like in the past I have had situations (especially on 70cm and
above) where the length of the feedline had a major effect on the difference
in the transmitted power at the antenna....as much as 50% less signal
measured between the transmitter and the antenna with random lengths as
opposed to only losing 4% or 5% when cutting the feedline to multiples of
the electrical 1/2 ~ .

A
"...if one tolerates bad manners, they grow worse. I must find the oaf
who did
this thing, explain to him his offense, give him a chance to apologise,
then
kill him." (R.A. Heinlein, "The Cat Who Walks Through Walls")

"Noon-Air" wrote in message ...
Instead of wasting bandwidth on stuff that cannot be changed by the general
populace (CW requirements) or stuff that is just to jerk peoples collective
chains (E-Bay posting in this NG), how about something that might even be
technical in nature??? How about the answer to this question??.....When
cutting the feedline for a dual band (2m/70cm), for maximum performance, it
needs to be cut to multiples of electrical half wave length. Does it need to
be cut for 2m? or 70cm? a combination of both? or split the difference?

Neither. You need the correct length of coax to reach from the antenna
to the radio. Anything else is pure voodoo.

Are you posting this in a swap forum just to jerk people's chains?

"Richard Cranium" wrote in message
om...
"Noon-Air" wrote in message
...
Instead of wasting bandwidth on stuff that cannot be changed by the
general
populace (CW requirements) or stuff that is just to jerk peoples
collective
chains (E-Bay posting in this NG), how about something that might even
be
technical in nature??? How about the answer to this question??.....When
cutting the feedline for a dual band (2m/70cm), for maximum performance,
it
needs to be cut to multiples of electrical half wave length. Does it
need to
be cut for 2m? or 70cm? a combination of both? or split the difference?

Neither. You need the correct length of coax to reach from the antenna
to the radio. Anything else is pure voodoo.

Is it??

Are you posting this in a swap forum just to jerk people's chains?

ummmm.....no

I have the formula to calculate feedline length for *monoband* V/UHF
antennas, and was hoping that somebody could shed some light on the dual
band dilemma either through calculations or personal experience or both.

I have the formula to calculate feedline length for *monoband* V/UHF
antennas, and was hoping that somebody could shed some light on the dual
band dilemma either through calculations or personal experience or both

Light has already been shed. The length of the feedline needs no formula.
If you insist on using a formula, try the following:

Measure the distance between radio and antenna, double it and then divide by
two.

"Best Performance", particularly on VHF and UHF usually means reducing
feedline losses. This is done in several ways:

1. Use the best quality, largest diameter coax you can afford.
2. Use the shortest length necessary.
3. Make the antenna resonant and impedance matched to your coax.

This last point may be the source of your misunderstanding. When the
antenna presents a resonant match to your feedline (1 to 1 SWR) the current
and voltage along the feedline don't vary; they are the same regardless of
where you measure them. Therefore it won't matter if the feedline is a
multiple of 1/2 wavelength...current and voltage are the same along the
feedline.

IF HOWEVER, your antenna is not well matched to the feedline, you will have
varying amounts of voltage and current along the feedline. The SWR will be
greater than 1 to 1. Under this condition, you can trim the feedline length
to improve the "apparent" match. This may allow your transmitter to put
full output power into the transmission line, and improve performance. But
the feedline will have increased losses because of the increased SWR. The
correct fix for this condition is not to trim the feedline to a particular
length, but to adjust the antenna for a better match.

73,

Roger K6XQ

Hi, in the 20 years I've been a ham almost any length worked OK. If you need
only 30 feet, there is no need to use 100'. There was one time though where I
had a lousy SWR on 440 and 2 meters was fine. I experimented with a few pieces
of coax jumpers. It truly did change the SWR. I cut a few inches off the main
coax and all was within the proper range. BTW, the proper way to check the SWR
is put the meter at the antenna. It's pretty hard to do this when the antenna is
20 feet in the air. At the radio is good enough.
Good Luck
73
------------------------

Noon-Air wrote:

Instead of wasting bandwidth on stuff that cannot be changed by the general
populace (CW requirements) or stuff that is just to jerk peoples collective
chains (E-Bay posting in this NG), how about something that might even be
technical in nature??? How about the answer to this question??.....When
cutting the feedline for a dual band (2m/70cm), for maximum performance, it
needs to be cut to multiples of electrical half wave length. Does it need to
be cut for 2m? or 70cm? a combination of both? or split the difference?

--
Steve @ Noon-Air Heating and A/C


When a work lifts your spirits and inspires bold and noble thoughts in you,
do not look for any other standard to judge by:
the work is good, the product of a master craftsman.
- -- La Bruyere

There are certain antennas that I have seen where the coax length is
critical - these are either special antenna configurations. One such
configuration is twin or multiple element antennas where the length of the
coax is used for optimal phasing. The other I have seen is for antennas
that are non-resonant and the coax is used as part of the matching network,
making it part of the antenna. I am not even sure why some antenna
manufacturers choose such designs, but I have seen them. This is less than
optimal for many reasons. One reason being, your coax becomes part of the
radiating element. This takes RF power away from the antenna - and puts it
on the coax, inside the vehicle or in the shack with you, where it can only
cause problems you dont need. I have seen a shorted coax (absolutely NO
power at the antenna) match up great to the radio because it happened to be
exactly the right length to the short.

Take a tip from me, if the antenna installation instructions say to cut
the coax to a specific length and it is not a phased, multi-element antenna
then stay away from it.

Hope that helps.

Wade

"Noon-Air" wrote in message
...
Instead of wasting bandwidth on stuff that cannot be changed by the general
populace (CW requirements) or stuff that is just to jerk peoples collective
chains (E-Bay posting in this NG), how about something that might even be
technical in nature??? How about the answer to this question??.....When
cutting the feedline for a dual band (2m/70cm), for maximum performance, it
needs to be cut to multiples of electrical half wave length. Does it need to
be cut for 2m? or 70cm? a combination of both? or split the difference?


--
Steve @ Noon-Air Heating and A/C


When a work lifts your spirits and inspires bold and noble thoughts in you,
do not look for any other standard to judge by:
the work is good, the product of a master craftsman.
- -- La Bruyere

How about radio sale ads, since that is what this group is for?

"Noon-Air" wrote in message
...
Instead of wasting bandwidth on stuff that cannot be changed by the
general
populace (CW requirements) or stuff that is just to jerk peoples
collective
chains (E-Bay posting in this NG), how about something that might even be
technical in nature??? How about the answer to this question??.....When
cutting the feedline for a dual band (2m/70cm), for maximum performance,
it
needs to be cut to multiples of electrical half wave length. Does it need
to
be cut for 2m? or 70cm? a combination of both? or split the difference?


--
Steve @ Noon-Air Heating and A/C


When a work lifts your spirits and inspires bold and noble thoughts in
you,
do not look for any other standard to judge by:
the work is good, the product of a master craftsman.
- -- La Bruyere