On Wed, 23 Feb 2011 21:53:00 +0100, "RadioWave" radio@oidar wrote:

Hi Richard,

Thank you for your reply and your interest in my homepage. I will answer
your questions between the lines.

I will do the same.

I will use the remarks that I get to improve the content of the article on
my site.

Hi OM,

I especially like your coverage of your antenna from I3VHF.

On your second page, unfortunately, you have some misconceptions about
loop antennas.

All antennas exhibit the same noise characteristics. If you erected
a conventional (electric) dipole in the same space, it would exhibit
the same characteristics.


I agree that they are electrically equivalents. However, my point is that
the magnetic loop has useful benefits over the dipole antenna for RX under
certain circumstances. I believe the magnetic loop construction will in many
cases deliver an acceptable signal at the receiver with less disturbances
such as atmospheric noise.

Hi Norbert,

Demonstrable proof shows otherwise.

About external noise sources:

The loop is smaller (less surface) and therefore picks up less static noise.

Static is indistinguishable from the RF you want to hear. In other
words static is RF, signals are RF. If your small loop picks up less
of one, it picks up less of both. However, this "picks up less" is
arguable.

The dipole covers a larger area in which there can be sources of noise.

Reread my statement: "If you erected a conventional (electric) dipole
IN THE SAME SPACE."

The pickup loop that connects the coax to the loop antenna is isolated from
the antenna and it forms a shortcut for DC. The signal transfer is
inductive.

This is a tautology, not a reason.

The magnetic loop tunes to the frequency and there is no external antenna
tuner needed.

Here, the Q of the tuned loop DOES contribute to less interference of
out-of-band signals. It does not reduce interference to in-band
signals. Noise is not specific to frequency, although single
frequency emitters can be called noise (unwanted).

About intenal receiver noise and mix-products:

The magnetic loop in itself is a band pass filter at the source of the
receiving signal. It eliminates strong signals outside the received
frequency. Therefore the receiver can receive the wanted signals with
maximum sensitivity. The band pass functionality of the loop protects the
radio from overloading. And as a result of that the radio will be quiet and
doesn't need to pick a weak signal from an overloaded band. The bandwidth of
the I3VHF is very small in the 40 m band. AM modulation is not possible as
the bandwidth of the loop is too small here for passing a standard AM
signal. The signal will be clipped and the transceiver react to that which
can be seen on the SWR meter. SWR starts to alternate on the rhythm of the
modulation. The bandwidth of the antenna gets larger in the higher bands. I
believe that in the 40 meter band the I3VHF only lets trough one frequency
in SSB. The receiver is almost mute tuning higher or lower.

Barring problems of lacking a choke on your control line introducing
SWR issues, I would tend to agree.

As for receiving the readability is more important than signal strength. The
lower RX signal from the magnetic loop is often more readable than when
using a full size dipole at ideal height. I think that the advantages are
best in the Low bands, e.g. 80, 40, 30 meter.

You have a lower signal because you have a lower antenna. Let's not
turn a deficit into a glowing recommendation - especially when you go
to transmit you lose that same gain from low height.

For TX there are advantages of the magnetic loop over the full size dipole.
When one has shortage of space. The high small band pass filter that the
Magnetic Loop is, makes the radiated signal free of harmonics. Therefore
there is a smaller chance of rfi to be expected .

A small (electric) dipole is identical in characteristics. It simply
doesn't come built with its own tuning mechanism.

Your arguments are not about antenna, but tuning.

Maybe some of the points here are not based on solid scientific research.
But it is what I found doing experiments with the loops.

It is quite curious how you describe a front/back ratio for a dipole
(the loop is a magnetic dipole, and as such "should" show a
conventional dipole pattern).


The data is based on the specifications of the manufacturer of the I3VHF
loop antenna.

http://www.ciromazzoni.com/English/L...oop%20Baby.htm

First thing I noticed was the loop on a tower.

In the manual, page 42, 43 there is a picture of the radiation pattern:

http://www.ciromazzoni.com/English/L...nna/Manual.pdf

It also surprised me as I expected a dipole pattern.

As for loop efficiency, you state:
"When a magnetic loop antenna is used for
3.5 MHz with a perimeter of 4 meter (13.3 foot) ,
it has an efficiency of approximately 3%."
Please show the math.

The 3 % efficiency is hypothetical based on the outcome of calculations
software that is available on the Internet.

I presume this is for the MIDI loop with a 2M diameter. The claim
offered is that it exhibits a Q of 1500 at 3.5MHz. The radiation
resistance for that size of loop is 0.49 Ohm. So, if 3% of the power
goes to 0.49 Ohm, then 97% of the power must go to heating up the
large tubular structure's Ohmic resistance (which would be very high,
and quite remarkable for that mass). Let's consider that you took an
Ohmmeter and measured half an Ohm in the structure, then you would be
losing only 50%, not 97%.

If you short your Ohmmeter leads together, I bet they have less than
half an Ohm resistance, why should this massive structure have more
loss than simple wire?

The argument would also have to answer the high Q (that much loss is
very low Q).

For example the loop calculation software of G4FGQ.

Give us the entry data and the formula.

73's
Richard Clark, KB7QHC