[Antennas] re Mosely classic 33

Tue, 01 Jul 2003 11:55:06 -0500

hmmmm,,

lore,,hmmm,,

feed point impedance 50 ohms,
you have a 1/4 wave of 75 ohm feed connected,

what's the impedance of the end of that feed line? hmmmm/

50 ohm antenna  1/2 wave of 75 ohm line, now what the impedance?

antenna that varies from 30 to 70 ohms (only a 1.6 to one SWR when desiring a 50
ohm load) what tribander is less than that at the band edges? not a whole lot.

30 ohm antenna  into 50 ohm line 1/2 wave will yield a 30 at the feed line end,
but a 1/4 wave length of 50 ohm feed, now at the end of that feed line what is
the impedance presented, it is NOT 30 ohms,

your feed line DOES effect the load presented to your transmitter, there is no
way it can not.

A10382 wrote:

> 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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