[Premium-Rx] ALR 25 Loop Information

[Dave Emery]

On Mon, May 03, 2004 at 04:36:29PM -0700, Doug Millar wrote:
> 
> 
> Dear All,
> 	Does anyone have any information about an Electro Metrics ALR 25 
> 	loop antenna and how well they work? For that matter how they work....Seems 
> that it has two BNC connectors.
> 	Doug K6JEY


	I am also curious, as I bought one on eBay recently.

	I am not completely without information, however, as I
researched the thing a bit before I bid.  

	They are a shielded magnetic loop antenna sensitive to the H
field component (magnetic) which is supposed to use a set of passive
approximately octave bandwidth transformers and matching networks to
match the round loop to 50 ohms over a couple of octaves frequency band,
with different transformers and matching networks for the other bands in
the 10 khz to 30 mhz range the unit covers.

	Mine is the manual select kind (the ALR-25M) with selection of
the matching network and operating frequency being done by a big band
switch on one side of the base with labeled positions for each band.

	There is also a ALR-25 version (no M) which has relays driven by
parallel TTL control lines to select bands (probably open collector
drive for either 5 volt or 12 volt reed relays).    This version has a
round military aircraft style connector for the control signals and I
think another connector for power.

	What so far is a mystery about mine is why it needs two TNC (not
BNC) connectors - one labeled signal and the other "INJECTION". I have
considerable difficulty trying to figure out what one would inject into
a passive loop and transformer matching network design. Best I can
figure is some kind of calibration signal.   (There is  unfortunately
another possibility - that the thing takes an external LO and mixes it
with the signal coming from the antenna to produce an IF - but the lack
of an obvious power source for this active circuitry makes me wonder if
it could be used that way).

	The antennas are designed to work with a specific 
Electrometrics  field strength receiver, but from what I was able to dig
up they seem capable of working with any 50 ohm receiver or spectrum
analyzer - at least the manual version does - the reed relay version
would require some drive for the reed relays in all probability.

	And for what it is worth I saw antenna factor numbers of around
12 listed...

	I was hoping mine might work as a calibrated magnetic field
antenna at HF, since the FCC BPL spec calls for magnetic loop field
strength measurements for part 15 compliance testing of power line
noisemakers.

	But I admit mine is sitting outside the door of my radio room
waiting for me to play with it... so I cannot report on how well it does
as a HF loop antenna, though I do have reasonably good calibrated
spectrum analyzers that work in that range and guess I can make some
measurements of it at some point.   But of course what counts for
utility as a HF antenna is not absolute signal output level but signal
to noise - loop antennas at HF have a reputation as quieter than long
wires in the presence of QRM from nearby electrical wires and
electronics.

	Oddly enough I worked as a consultant to the company that owned
Electo Metrics for a while a few years back, but unfortunately the
parent company is long gone and I don't know who if anybody wound up
with the Electrometrics operation or whether they still exist.

-- 
   Dave Emery N1PRE,  die at dieconsulting.com  DIE Consulting, Weston, Mass 02493