In article ,
Bruce W.1 wrote:

That's the part that's difficult to measure. Some of my good HT
antennas that came with the radio, when tested alone on a good ground
plane, have lousy SWR. Yet these radios are fully capable of being
connected to an external antenna.

A perfect quarter wave is 36 ohms. Yet HT's are happy driving an
external 50 ohm antenna.

Well, for one thing, the transmitter "sees" the total load - the
quarterwave's radiation resistance, the radiation resistance of
whatever is servicing as the ground plane / counterpoise / other half
of the dipole, and the losses of both. The role of GP/counterpoise is
usually taken by a combination of the HT's metal body, and the user's
skin (capacitively coupled to the HT body). The radiation resistance
and losses of the HT body and user's skin are difficult to predict,
and no doubt vary all over the place depending on the situation, sweat
levels, phase of moon, etc., but I imagine they're high enough to
boost the total feedpoint impedance up well above 36 ohms.

For another things, HT finals _have_ to be reasonably tolerant of poor
matches, or the failure rate would be astronomical - there's just too
much variation in antenna feedpoint impedance to allow such devices to
be designed with finicky, sensitive finals. I would guess that modern
HTs all use internally-ballasted multiple-emitter RF transistors which
are very conservatively rated (e.g. voltage breakdown ratings of
several times higher than the power supply voltage, plenty of excess
current-dissipation margin due to the ballasting) and may include
automatic high-SWR power foldback circuitry as well.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

On Wed, 06 Apr 2005 02:37:12 -0000, (Dave Platt)
wrote:

In article ,
Bob Miller wrote:

For example, the rubber duckie that comes with my 2 meter Icom T2-H,
at 147 Mhz, has an swr of 3.0, and an impedance of just under 400
ohms, according to my MFJ 269 antenna analyzer. But the antenna works
fine on the handie talkie.

You might want to try repeating the measurement with the MFJ 259 held
up beside your head as if it were an HT.

I've been told that HT duck antennas are often tweaked for best match
and lowest SWR when used in this position, with a significant amount
of capacitive coupling to the user's head. Measurements of a couple
of 'em with my 269 seemed to support this - they "read" a lot better
in a "typical HT use" position than they do if the analyzer is being
held out at arm's length.

Hmm, maybe I don't have as much upstairs as I thought -- moving my
head towards the antenna doesn't do much -- but moving my hand towards
the antenna does affect things -- this is all highly approximate
measuring with the MFJ 269; guess it's a miracle these HTs work as
well as they do... touchy little devils to measure...

bob
k5qwg