Topband: Bad Frequencies and Loop Pre-amps

n4is at n4is at
Tue Nov 13 08:57:08 EST 2018

The main issue with IMD is the passive IMD generated by oxidation between
two metal structures or wires. The IMD's on 1810,1820, 30 40 50 etc, most of
the time is irradiated on your own back yard from things like.

1- Aluminum copper oxidized contact.
2- Mast not grounded to the tower, it needs a flat cable to ground it, rotor
is not a grounding point.
3- Rust bolts on towers.
4- Bad overheated  BALUN on low band antennas.
5- House wires connected direct to diodes on security devices power supply,
like cameras or motion detectors.

I don't have any nearby IMD on 160m during the night. Sometimes a weak
signal come and go during  day time.

About the remote preamplifier, please don't believe on most project
construction pictures you see on internet using plastic box, open frame
relays, coaxial open at the end soldered on connectors.

The real issue here is to amplify only the signal received from the loop and
not add common mode noise from the cable, DC line, and the circuit itself if
not shielded with a good metal box. 2000 ft of cable is a good antenna even
if grounded 10 inches or more.

Choke is a "must" but does not fix everything. Balanced feed line does help
a lot, you can parallel to coaxial cable, ground the shield and use the two
center conductors. Like the center connector of two RG58 give you 100 ohms,
two RG8 150 ohms, then you need a good BALUN and lots of shield. Best is
unshielded twisted pair, no common mode ground.

No pain no gain!


-----Original Message-----
From: Topband <topband-bounces at> On Behalf Of Roger Kennedy
Sent: Tuesday, November 13, 2018 8:25 AM
To: topband at
Subject: Topband: Bad Frequencies and Loop Pre-amps

I have never received ANY harmonics or intermod products from any of our
British Medium Wave (AM) broadcast stations.  That's at several different
QTHs, some of which were quite near the transmitters. (and never heard of
anyone else having issues)

They have to meet VERY high specs . . . but also, most are less than 1kW !
(one of the stations I worked for had such a good Tx site - a salt marsh -
they had to turn the power down to 150 watts!) Even the very big ones are
usually only 10kW.

So no need to avoid the 9kHz harmonics as far as us Brits are concerned!

What IS a problem is all the Russian Navigation Beacons in the Baltic that
pretty much wipe out everything between 1810 and 1820 kHz, so please try and
avoid that part of the band if you want to work any EU !  (they sometimes
turn them all off . . . but they are on most of the time.)

The pre-amp I use on my 6ft 160m receiving loop is actually the design MFJ
use in their 1026 noise canceller (I have a 1025 but copied their preamp
design, as it uses the same pcb) . . . I get no intermod during contests,
even though many EU stations are 45dB over S9 !

Roger G3YRO
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