Richard,

I think you are confusing the Isotron by Bilial with the Isoloop
formarly made by AEA. The Isoloop was a small loop antenna tuned with
a large remote variable capacitor like you describe. The Isotron is
just a large fixed capacitor (you can make a slight manual tune to it,
not remote tune like a magloop) and a muliturn solenoid inductor. I
serves as nothing more than a lossy load to dump your power into. The
actual antenna is the metal mast you are supposed to attach the
Isotron to. The antenna is essetially a small radom wire antenna
composed of the metal mast and/or the outside shield of the coax
attached. Whatever power is not lost in the Isotron, travels on the
metal mast and outside shield of the coax and couples into your house
wiring, television and neighbors appliances, and oh yes, a small
amount is radiated. It works essetially the same as the supposed
cross field and eh antenna. A large LC tank connected to a short
random wire antenna composed of the ground lead and outside of the
coax shield. If you choke off the current from the ground lead and
outside coax shield, you essentially have an unshielded dummy load.
Thats all there is to them.

73,
Dan

(Richard Harrison) wrote in message ...
Henry Koleanik, WD5JFR wrote:
"Anyone really know theory behind these things?"

The Isotron is a small tuned loop. Tuning gets rid of reactance which
would cripple performance were it not removed from the loop.

A small loop has a null perpendicular to the plane of the loop. Energy
suppressed in directions of the loop`s axis is radiated in the plane of
the loop giving a directivity power gain of about 1.5 (not dB, where
it`s just under 2 dB gain) over an isotropic radiation in the plane of
the loop as in other directions as well.

The bad news is that even made with a large surface area, the small
loop`s loss resistance looms large as compared its radiation resistance.

The 3rd edition of Kraus` "Antennas" is a clear source of single-turn
circular loop information. Fig.7-10 on page 209 gives radiation
resistance versus loop circumference. Fig. 7-11 gives directivity.

If radiation resistance were 0.5 ohm and r-f resistance were 0.5 ohm,
efficiency would be:
0.5/1=50%

Kraus has Fig. 7-17 on page 217 which gives radiation efficiency as a
function of frequency for a 1-m-diameter single-turn 10mm copper tubing
loop in air. At 1 MHz, the loss is about 40 dB. At 10 MHz, the loss is
about 6 dB. The radiation resistance of the loop is rising more rapidly
than is the loss resistance as frequency goes up. The loss is enormous.
This is ok for reception in most cases, but it`s very dear for
transmitting.

Best regards, Richard Harrison, KB5WZI

Dan, WK8L wrote:
"I think you are confusing the Isotron by Bilal with the Isoloop
formerly made by AEA."

I apologize for causing confusion. The Isotron is only a large capacitor
with a trimmer?

A large capacitor has a low reactance and to form a resonant circuit.
must be paired with an equally low reactance of the inductive variety. A
small rod or a small loop, in terms of wavelength, have similarities.

Terman is much more eloquent than I so I`ll quote from page 907 of his
1955 edition:
"The directional pattern is independent of the exact shape of the loop,
provided the loop is small compared with a wavelength.

The directional pattern of a small loop is identical with that of an
elementary doublet. The only difference is that the electric and
magnetic fields are interchanged. For this reason a small loop is often
called a magnetic
doublet.

The radiation resistance of a loop antenna is less the smaller the loop
area. For the radiation resistance to be large enough to give good
antenna efficiency, it is necessary that the loop perimeter be of the
order of a wavelength."

So the name "magloop" may come from "magnetic doublet". When is a loop
small enough to be a 'magloop"? One requirement may be the same current
in all sides of the loop and I`ve seen that specified as a perimeter of
0.1 wavelength or less. Like many specifications it may be arbitrary.

I suppose calling a capacitor an "Isotron" is arbitrary too.

Best regards, Richard Harrison, KB5WZI

In article ,
Richard Harrison wrote:

I apologize for causing confusion. The Isotron is only a large capacitor
with a trimmer?

Well, from what I can see of it (based on the diagrams on the Web), it
looks rather as if it's intended to behave like a very short dipole.
It's center-loaded and resonated with a large coil, and has a really
humongous capacity hat (the plates). The center coil, and the short
conductors leading from the top and bottom of the coil to the two
capacity-hat plates, would do the radiating.

If this is a correct description, then I'd expect the Isotron to
suffer from the same limitations as any short, loaded dipole - a low
radiation resistance (perhaps only an ohm or two) and a relatively
high loss resistance. Used in isolation (e.g. on an ungrounded mast,
with a choke on the feedline) its efficiency would be quite poor.

The fact that it's stated that it _must_ be grounded to a metal mast
for proper operation raises a big red flag (in my own humble view, at
least). This suggests that other folks are correct in asserting that
the mast, and the outside of the feedline are doing a lot of the
radiating.

It's even possible that in this configuration, the antenna system does
its best radiating when the Isotron itself is tuned _away_ from a good
match with the feedline, thus forcing significant currents onto the
feedline and the mast. One might (in this case) even view the Isotron
assembly as being something akin to a transmatch, with its reactivity
matching the impedance of the "antenna" (the mast and feedline) to the
feedline.

It'd be quite interesting to have somebody set up an Isotron as
directed, tune it to get the best signal in and out, and then run an
RF current meter over the mast, mast-grounding wire, and feedline when
transmitting. If there are relatively high currents present on
these conductors, it'd certainly be diagnostic!

--
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!

This suggests that other folks are correct in asserting that
the mast, and the outside of the feedline are doing a lot of the
radiating.
============================

What's wrong with radiation from the feedline? It's just as useful as
radiation from elsewhere.

On Tue, 23 Mar 2004 19:44:47 +0000 (UTC), "Reg Edwards"
wrote:
What's wrong with radiation from the feedline? It's just as useful as
radiation from elsewhere.

It's like promoting your SUV enjoying 80 MPG (but failing to disclose
that is with a 100 MPH tailwind).

Reg Edwards wrote:
What's wrong with radiation from the feedline? It's just as useful as
radiation from elsewhere.

Maybe for an Isotron, but not for a beam.
--
73, Cecil http://www.qsl.net/w5dxp



-----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
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What's wrong with radiation from the feedline? It's just as useful as
radiation from elsewhere.

Maybe for an Isotron, but not for a beam.

Or in cases where radiation coming off of the feedline would be
inconvenient for other reasons - e.g. the feedline runs in close
proximity to other wiring (RFI and telephone interference, or signals
getting where they don't belong in a multi-repeater shared site), or
metal pipes or metal-loaded walls (e.g. stucco).

--
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!

"Reg Edwards" wrote in message
...
This suggests that other folks are correct in asserting that
the mast, and the outside of the feedline are doing a lot of the
radiating.
============================

What's wrong with radiation from the feedline? It's just as useful as
radiation from elsewhere.

If I have a nice yagi array aimed at the moon and thus a low system noise
temp, and the feedline is allowed to radiate, I would think that might have
a bit of an effect not only on the array's pattern but also ruin the system
noise temp. The return loss then is also a function of cable length, routing
etc. Hardly a predictable array.
I can think of other examples. Although I admit in the case of the Isotron,
cable radiation is probably an advantage.

Dale W4OP

Reg Edwards wrote:
This suggests that other folks are correct in asserting that

the mast, and the outside of the feedline are doing a lot of the
radiating.

============================

What's wrong with radiation from the feedline? It's just as useful as
radiation from elsewhere.



If your interest is to spray your rf all over the place then a radiating
feedline is really FB.

If you have a $1500 tower, a $600 rotator, a $800 beam, a $3000
amplifier all connected to a $3000 transceiver, then you really want to
steer your RF in a controlled direction. Under these conditions a
radiating feedline destroys the nature of the antenna pattern and you're
not getting what you paid for.