In article ,
dave wrote:

Telamon wrote:
In article
,
" wrote:

On Jun 29, 7:25 pm, Telamon
wrote:
In article
,



" wrote:
On Jun 28, 6:17 pm, Telamon
wrote:
In article
,
" wrote:
On Jun 25, 5:29 pm, "Justtis" wrote:
Anyone know of a good site offering build plans. Prefer plans from
someone
who has built and used same.
I need a quiet antenna like this and Wellbrook is just too
expensive!http://www.wellbrook.uk.com/index.html
I use the ALA100 with home brew loops. Damn good, but you are right
that it is expensive. The ALA100 is the cheapest of the Wellbrook
products. They have some variants of it now. You need a good radio to
use an untuned loop since it delivers a huge amount of RF.
SNIP
So stop whining about the cost and build a low cost passive loop. It if
works well but you still need further improvement it will at least tide
you over.
All the amplification does for you is allow the loop to be smaller,
which may mostly be a convenience depending on the environment.
Be sure to build a balanced loop that only responds to the magnetic
field.
I saw your other post on the subject. Feeding a loop to a 50 ohm input
of a radio will not work well since once side of the loop will be at
ground.
It works very well. I have several loop antennas.

This is where the wideband transformer comes into play. You
can float the loop above ground if you have the transformer. It is
possible to do the same with a differential amp, but good transformer
would be lower noise.
The transformer is not necessary for a connection to the coax or a
balanced amplifier input.

You could clamp a few ferrite cores around the coax lead-in to the radio
at the antenna connection to block noise coming from the radio if you
don't already have the radio plugged into a EMI noise suppression power
strip.

Er, is there a reason you don't want to do this correctly?

You don't need a transformer to "do it" correctly.

The thing with shortwave is you can toss out a piece of wire and get
something. It is all a matter of digging the signal out of the noise.
In any event, the transformers are easily purchased on ebay. Or in
surplus shops if I don't beat you to them.

Yeah, that's what people do and then post complaints here.

I'll let you in on a little secret. In order to be successful you have
to remove obstacles not create them. You don't need to transform an
impedance that close to the coax impedance.


BalUn?

The transformer type would be a BALUN but it is not needed.

The loop and lead in are all made of the same 50 ohm coax. The shield
of the lead in and both sides of the loop are all connected together.
The shield is opened halfway around the loop so the shield is balanced
around the loop. One side of the loop center conductor is connected to
the lead in center conductor. The other side of the loop center
conductor is connected to the joined shield connection.

The split shield picks up the far electric field equally and at
opposite polarity so it is canceled. The coax center conductor has
current induced in it from the magnetic field since it is shielded from
the electric field. For the size of the loop the output will be less
compared to an antenna that uses both fields to generate a signal to
the radio but it also does a better job of canceling local noise
sources provided the noise source is far enough from the antenna that
its electric field will be canceled by the balanced shield.

The result is a generally lower noise floor. Since I live in suburbia
noise is a bigger issue than picking up the signal.

In reverse order of preference:

1. Common mode antennas are near worthless around here.

2. Hertzian voltage antennas perform a little better than common mode.

3. Any type of hertzian closed loop antenna performs better than the
voltage type.

4. The hertzian balanced shield loop antenna works better than 1 thru 3
above.

You will notice a trend in a certain antenna parameter in the order of
preference above. I'll leave it as an exercise for the reader to
discover it.

--
Telamon
Ventura, California