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G0FTD > TECH 16.09.04 00:22l 48 Lines 1719 Bytes #999 (0) @ WW
BID : A16913G0FTD
Read: GUEST OE7FMI
Subj: Re: A Nest of Dipoles for HF.
Path: DB0FHN<DB0RGB<OK0PPL<DB0RES<ON0AR<ZL2TZE<GB7YFS<GB7CIP<GB7SXE
Sent: 040915/1204Z @:GB7SXE.#38.GBR.EU #:5198 [Hastings] FBB7.00i $:A16913G0FTD
From: G0FTD@GB7SXE.#38.GBR.EU
To : TECH@WW
Just a couple of small observations if I may.
There's nothing wrong with the concept dipole nests BUT it can often
be misunderstood by it's users.
Firstly it's true that you'll be "chasing your tail" if you think you're
going to be able "swr" each antenna for each band without effecting the
match of the others.
Secondly when this sort of idea came about we were all using valve PA's,
and the small variation in SWR was easily taken care of with the tank
circuit (plate'n'load for the newbies). As such the idea is a great one.
Today's solid state PA's would generally force a user to assume that a
good match was obtained easily in a nest system - which is rarely the
case. It may only be something like 2.1:1 but it's enough to force
the protection circuits in a modern rig to want to start closing down.
In this case a simple ATU at the rig end is acceptable (but never in my
opinion in any other case!).
Losses will be minimal through SWR although it's worth considering the
*additional mismatch loss of the cable* on a long length in *addition* to
the actual line loss when matched.
On a long length of coax and a high swr, you could have as much as 8db
*line mismatch loss* as well as the loss in efficiency due to a high
SWR !
Remember an SWR of 3.1:1 represents 25% (approx 1db loss).
The mismatch line loss is a different kettle of fish altogether, and that
could be as much as 8db in a typically bad case !
It's unfortunate that the latter receives scant attention in the amateur/cb
community - and everyone assumes the lack of "dx" is due purely to the
antenna SWR.
- Andy -
Bring back the Old Grey Whistle Test.
Bring back the Great Egg Race.
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