In article 0P0_d.698179$Xk.229269@pd7tw3no, tom wrote:

How about if I put the stripped-back coax antenna inside a waterproof
plastic bag that went over the top of it?

The bag would have to be 24" long. You need to keep not only the very
top of the stripped section dry, but the whole area where the braid
comes out of the stripped insulation and is then turned around
downwards.

I think that would certainly help
keep it dry, but would the plastic bag create any kind of mutual-inductance
problems, or does that only happen with conductors or materials with hi
dialectric coefficients?

A thin plastic bag would have little effect on the antenna's
characteristics - the layer of plastic dielectric is too thin to
change the behavior significantly.

The bag would probably make the antenna rather more prone to flap
around in the breeze, and that'd probably cause much more of an effect
than the bag itself would.

I remember reading that a the larger the diameter of the radiating element,
the slower the propogation velocity, so I agree with you about how it might
screw up the normal calculations --- but by how much, though?

At a wild guess, a few percent.

Given that a
vhf dipole has an 11mhz wide band within which the swr is below 2, does it
matter if the resonant frequency is off even if its off by a mhz or two?

Can you cite the source of that figure? Seems a trifle wider than I'm
used to seeing.

If I wound the feedline itself into an 8 turn, impromptu balun and slid a
ferrite bead on the feedline just below the end of the dipole part as a
further barrier to return RF, wouldn't that basically eliminate any
significant return current issues?

Possibly so. Quite honestly, a small amount of RF on the feedline is
not likely to cause you any problems. It'll alter the antenna's
pattern somewhat but since you're dealing with a wide-pattern omni I
doubt that you'll notice any significant change in the real-world
performance.

Am I missing anything, or can I basically count on a setup like this one to
not stress my final? If not, why not?

Maybe yes, maybe no.

Issue 1 is how well the antenna actually tunes up on your frequencies
of choice, which will depend on a lot of factors which are difficult
to predict in advance. For example, whether the folded-back braid is
stretched downwards along the coax insulation, and how tightly, will
probably alter the effective length of the sleeve by as much as an
inch or so and could significantly shift the SWR. Nearby objects, the
length of the feedline (if there's RF on the outside of the coax),
etc. can also affect the tuning. It's a lot easier to try, measure,
cut, and try again than it is to predict the behavior with the desired
degree of precision.

Issue 2 is how well your radio deals with higher-SWR conditions. Some
rigs seem to have active load-mismatch-detection circuitry and an
active power-reduction capability. Others seem to depend on having
rugged, conservatively-rated RF power transistors, operated well below
their safety margins, which are capable of standing a lot of abuse.
Others just emit blue smoke and give up :-(

Hence, it's likely to depend on your rig and your antenna.

Frankly, if you're concerned about the risk of damage, I would
encourage you to [1] build a design such as a ground plane which uses
relatively rigid parts which won't move around during use and thus
won't be likely to de-tune, and [2] find somebody who has a VHF-rated
antenna analyser, and borrow it for the half-hour needed to trim your
antenna to a low SWR. I'd think that any ham club with a couple of
dozen active members, is likely to contain at least one member who
owns an MFJ 259 or similar and is willing to Elmer you on a simple
antenna construction-and-tuneup project.

--
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!

Yeah, maybe I should just try to get some assistance with an swr meter
rather than endlessly screwing around, wondering how much I'm off resonance,
and all the stress associated with all of that. Maybe, if nothing else, the
shop where I'm buying all my gear might have one.
In any case, thanks for all the advice --- I'm pretty clear on whats what,
now.

--
73
Tom H
VA7FAB

On Wed, 16 Mar 2005 22:17:47 GMT, "tom" wrote:

Yeah, maybe I should just try to get some assistance with an swr meter
rather than endlessly screwing around, wondering how much I'm off resonance,
and all the stress associated with all of that. Maybe, if nothing else, the
shop where I'm buying all my gear might have one.

If not, http://www.mfjenterprises.com has vhf swr meters in the 35
dollars u.s. range and up. You'll need one eventually...

bob
k5qwg


In any case, thanks for all the advice --- I'm pretty clear on whats what,
now.

They don't cover 2m, I checked.

--
73
Tom H
VA7FAB

On Thu, 17 Mar 2005 02:52:07 GMT, "tom" wrote:

They don't cover 2m, I checked.

You didn't check far enough. The MFJ-812B covers 2-meters and 220.
It lists for $34.95.
http://www.mfjenterprises.com/produc...rodid=MFJ-812B

Dick

You're right --- I only looked at that other site. I'll definitely shop
around for one, but even if I can find one, it'll be $70 with taxes, in
canadian dollars, and it isn't likely that I'll be able to find to begin
with up here, but what the hell --- now I know that there relatively
inexpensive meters I'm further ahead than I was before I knew about them. I
thought they were all at least $150 and up. Thanks.

--
73
Tom H
VA7FAB