From: firstname.lastname@example.org (Tom Bruhns)
Subject: Re: Should one worry about reflected current into untuned loads at low power?
Date: 9 Dec 2002 11:36:46 -0800
References: <email@example.com> <firstname.lastname@example.org> <email@example.com> <firstname.lastname@example.org>
NNTP-Posting-Date: 9 Dec 2002 19:36:46 GMT
Paul Burridge wrote in message news:...
> On 8 Dec 2002 02:28:22 -0800, email@example.com (Tom Bruhns) opined
> >As for Paul's question about a recommended transistor: you didn't say
> >as I recall whether it's to be a linear amplifier or a class-C stage,
> >and if it's class-C whether it's to be amplitude modulated or not.
> I thought class C would be the best way to go, really. The input
> signal is already FM so an additional power stage shouldn't affect the
> signal information adversely as it would had it been AM or SSB. We're
> just simply seeking to up the power of a carrier wave, effectively.
Yep, a simple class C stage should do fine. In fact, you may find
that a transistor designed for collector-modulated AM CB band use
would do the trick for you, but you're on your own to verify the
suitability of any particular one.
> Well, maybe there's a simple network one can build to match even a
> random length of wire?
You bet. To begin with, you should probably pick the "random" length
to be something reasonable. If you make it very long, the radiation
pattern will have an incredible number of lobes and nulls. Try to
keep it less than a wavelength. You MUST have somewhere for the
return current to go...some groundplane or counterpoise. A simple "L"
network can match a wide variety of loads, though it helps to know
ahead of time roughly what the load is. Avoid loads which are highly
reactive or are resistive but a big ratio to your nominal transmitter
load impedance. If the transmitter expects to see a 50 ohm load, use
a (50-ohm) SWR bridge to adjust the "L" network for minimum SWR. The
more reactive your load (as a percentage), or the higher the ratio
between antenna resistance and desired resistance, the sharper and
more difficult the tuning will be. So things in the vicinity of 1/4
wave or 3/4 wave against a decent ground should be fairly easy to
match, and things near 1/2 wave or 1 wave resonance are likely to be
problematic. A large diameter "wire" (e.g., a piece of aluminum or
copper pipe) helps to keep the impedance "reasonable."