It does not take much RF in the shack/house to cause problems. Keep
the RF outside.
An effective balun goes a long way to avoid these problems.
Noise from the house couples to the antenna if there is not an
effective balun, raising the receiver noise level. Using an effective
balun can lower the receiver noise levels.
A good balun will have negligible loss (by design).
Operating Coax at high SWR (a la G5RV) can have significant loss.
Even a fairly short length. Run the feedline modelling software that
comes with the ARRL Antenna Books to quantify. Try to keep high SWR
coax lengths to several feet at most.
At least one company (DX Engineering) makes baluns that will handle
high power. Take one apart and inspect it, they are well made and
have a lot more ferrite than most.
Operating a balun with SWR is not a problem as long as the ratings
are not exceeded. The voltage and current are larger by as much as
the square root of the SWR, so a 10:1 swr will increase
voltage/current by as much as the square root of 10, or about 3. Use
a balun with adequate voltage/current ratings for the power and SWR.
A 4:1 balun current balun splits its ferrite in half for the two
internal 1:1 sections. So it has half the choking reactance of a
straight 1:1 for the same cost (ferrite quantity). Generally a 1:1
current balun is a better choice for high SWR use. Let the tuner do
the matching, allow the balun to do the balancing.
A voltage balun doesn't keep the RF out of the house, so is not of
much use here. Most 4:1 baluns are voltage type, so you have to shop carefully.
As to the original question about the balun failure, the ineffective
balun may have allowed considerable RF onto the ground system and
conducted into the stereo. The RF gets rectified and the stereo
attempts to amplify the resultant frequency components within its
passband (some possibly out of audio range). The high power
consumption pops the fuse.
Using balanced antennas and a quality effective balun keeps the RF
out of the house.
73,
-- Alan, wb6zqz
At 06:23 AM 5/15/2007, Brett gazdzinski wrote:
What is the benefit of a balun?
I cant run them, as I run legal limit AM which
will toast any balun I ever heard of.
At 100 watts, or QRP, RF in the house cant be an issue,
I don't have problems with the AM and the G5RV right over the house.
I would think it would just add more loss.
I used to have resonant 80 and 40 meter dipoles, but took
them down and built a home made G5RV, #12 wire for the open wire
line, and only about 20 feet of RG214 coax into the tuner
or directly into the K2 with its built in atu.
I will have very high swr on both the open wire line
(does not matter as there is almost no loss),
and the RG214, but there is only 20 feet of that, so I would
guess the loss is slight.
Now I could fit a balun if I just used the K2, but what
would be the advantage?
I hear you should never run a balun with any swr, the losses
go way up. I think they now make tuners with the balun
before the tuner (rig, balun, tuner, antenna) that works
well....
Brett
N2DTS
>
> The following happened to me a few years ago as a brand new ham in my
> pre-Elecraft days and I wonder if anyone can explain to me
> the details. I
> apparently had high swr and exceeded the rating on my balun.
> It suffered
> accordingly but didn't completely fail. The result was when
> I transmitted cw on
> 40m an internal fuse would blow in the stereo which was on in
> the next room
> room. It took two go rounds before I caught onto the cause
> and effect... A new
> & better balun cured the problem.
>
> At the time I had a Zepp (135') and 4:1 balun at the house
> entrance. I assume
> the ruined balun was emitting all sorts of RF that the stereo
> amplifier picked
> up, tried to amplify, and couldn't. But I don't understand
> exactly what would
> make it draw that much current. Can someone enlighten me?
>
> By the way, since then I put up a dipole, halfwave for 160m,
> about 200' behind
> the house. That's what I should have begun with.
>
> Thanks!
> Mike ab3ap
_______________________________________________
Elecraft mailing list
Post to: [email protected]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com
