From: email@example.com (carltons)
Subject: Re: Should one worry about reflected current into untuned loads at low power?
References: <firstname.lastname@example.org> <email@example.com> <firstname.lastname@example.org> <email@example.com> <firstname.lastname@example.org>
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NNTP-Posting-Date: Tue, 10 Dec 2002 10:04:34 GMT
Date: Tue, 10 Dec 2002 10:04:34 GMT
In article <email@example.com>, firstname.lastname@example.org
(Tom Bruhns) wrote:
> Paul Burridge wrote in message
> > On 8 Dec 2002 02:28:22 -0800, email@example.com (Tom Bruhns) opined
> > thusly:
> > >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."
I'll buy all this about the random wire especially the use of an L match.
It is the most efficient and will get more of your power to the load with
the least amount of circulating currents. With an L match, the higher
impedance always has the shunt element across it. If you need
omnidirectionality, then a coaxial sleeve antenna made from helically
wound elements is for you. K4KEY (Doug) can probably help you. He has
done a lot of commercial work at these frequencies. You can find him in
the radio amateur call book.