[Antennas] re Mosely classic 33

Tue, 1 Jul 2003 12:34:37 -0400

Ahhhh...changing feedline length - part of the 'legend and lore' of radio
comm.  This is the stuff that makes for very interesting and lively e-board
threads.

It's been my experience that If changing the feedline length changes the
VSWR then - generally - the feedline has electrically become part of the
antenna radiating element(s) -- and is radiating....  This is a
generalization.

This may or may not be desirable for YOUR setup.

This was a part of the 'CB' culture that was rife with poorly designed and
cheaply made antennas, coax, and at times, amateurish installations.
Manufacturers frequently sold 'packages' (radio, antenna, coax, mounting
brackets) that were not engineered well as a complete system.  Saving 2
cents per foot on the coax (typically a savings of $0.34 for the whole
package) made someone in marketing a real hero !

Many radio shops (especially those that do mobile installs), found years ago
that changing the feedline length resolved some installation problems and
helped speed up customer service.  Some of the problems were of the
installer's making and some of the vehicle/antenna/radio's characteristics.
Of course, having half the energy radiating INSIDE the trunk really did not
do the customer any real good.

Ideally, the feedline (especially unbalanced line like coax) should NOT be
part of the antenna as you do want all the rf energy radiating at the
antenna, not in the trunk, house, attic, or shack.  However, there are SOME
antenna installation designs that actually rely on the feedline radiating -
usually using ladderline/open-line as the transmission feed.

The best argument for using half wave multiples of feedline length is, as
Rick noted, to allow you to place the meter at the radio and have a fair
idea of what the situation is at the antenna.  Most users fail to calculate
the velocity factor of the coax 0 something that should be gotten from the
manufacturer for THEIR product and not out of a book - whose spec may vary a
bit by manufacturer. For HF, an extra 50' of unneeded coax is a waste of rf
energy (think in terms of dB loss per 100').
=====================
73, Frank
 ._._.

----- Original Message -----
From: "RICK COLE" <[email protected]>
To: <[email protected]>
Sent: Tuesday, July 01, 2003 12:42 AM
Subject: [Antennas] re Mosely classic 33

you cannot change the vswr of the antenna by changing the feedline
length...what is happening most likely is you are bringing the feedline
length close to a multiple of 1/2 wave length at the operating frequencey
and therefore getting a mirror-image of the vswr at the feedpoint showing at
the meter in the shack...don't forget the correct place to check the vswr is
at the the antenna itself and not in the shack but sometimes this is not
physically possible so the best thing to do is get an exact image in the
shack end by having the feedline in multiples of 1/2 wavelength to enable
this...this is an electrical length not physical length length and the
formula to calculate this is by 468/F xVF where V is the Velocity Factor of
the cable in use and F is the Frequency in use
Regards  Rick

Try a different length of feed first, (put in like a 5 or 10 foot patch
cord) and
see what happens,,

Mine was resonant no place many years ago and the guy at mosley said to try
that
and like WOW great SWR right where it should be.. He said sometimes ya get
weird
lengths.

Joe wb9sbd

[email protected] wrote:

> I have a Classic 33 that is behaving strangely. Although it is cut for CW,
it
> resonates very high in the phone bands on 20-15-10.
>
> I recall, years ago, someone put together a diagnostic guide of possible
trap
> problems and their symptoms. Does anyone have such a document they can
share?
>
> Tom, K5RC
> Virginia City NV
>
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