----- Original Message -----
Sent: Friday, January 13, 2017 11:29
Subject: [CrossCountryWireless] HF
Magnetic Loop Antenna - prototype under test
We've been making the HF Active Loop Antenna for a few years now and
whenever we attend radio rallies and hamfests or mention it to other radio
amateurs I'm always asked "When are you going to make a transmit
Over Christmas and New Year I've been laid low with shingles so I've taken
the opportunity of enforced idleness to research and design a magnetic loop
antenna for HF.
The first thing to note is that there is an awful lot of incorrect and
misguided information on the internet about magnetic loop antennas. Since I
built the prototype I can understand how that comes about as the resistive
losses have to be kept extremely low and it's possible with a small amount of
loss (say 0.1 ohm) to have an antenna that appears to work and match correctly
but has high RF losses giving misleading results.
The first prototype uses a 1m diameter loop of S400 (LMR400 equivalent in
the UK) with high quality N type male connectors. These fasten to two N type
female connectors mounted on one of the polycarbonate boxes used for our
This allowed me to test various feed arrangements and a range of variable
The first capacitor was an Oren Elliot 384 pF single gang variable. This
gave very poor results on transmit but appeared to tune and peak perfectly on
receive. A DC check showed 0.1 ohms across the capacitor frame and sllding
contact to the rotor. This was enough to lose a lot of power on transmit.
The second capacitor was a new old stock Jackson Brothers 365 pF dual gang
capacitor. The ends of the loop were connected to the stators allowing the
rotor to tune without any RF current flowing through the sliding contact. This
was efficient but the maximum power that could be applied was 25W before the
capacitor flashed over. I'd like to manufacture an antenna that could take
100W as least.
The supply of the JB capacitors is limited not being manufactured anymore
and the Oren Elliot range of capacitors are extremely expensive once they have
been shipped to the UK and had tax applied.
At this point it's worth noting that the magnetic loop antenna is a
resonant tuned circuit and the RF voltages across the capacitor and the RF
current flowing through the loop should be very high if the losses are low.
With 100W on 14 MHz the RF voltage across the capacitor should be 2579 V rms
and the RF current in the loop 16.6A. Serious stuff!
There are very few variable capacitor manufacturers left so the choices are
very limited and expensive.
At this stage the only option was to look at making a capacitor
The first prototype is a compression type capacitor using PTFE sheet as a
dielectric. This should be capable of withstanding the high RF voltages and
The first test of the PTFE capacitor worked well. The tuning bandwidth is
sharper than the air spaced capacitors indicating that the losses are
Applying RF power to it there was a dramatic flashover at 80W. After
stripping it down the flashover was not across the capacitor but from one of
the mounting screws on the N type sockets. The arc had jumped across the air
gap to the capacitor plate.
That's the progress so far. The flashover indicates that the RF losses in
the antenna are very low allowing high RF voltages. The RF matching works well
giving a VSWR of 1:1 at resonance.
Before it goes into production it needs work on the safety aspects
insulating the N type connectors and replacing the mounting screws with
polycarbonate ones for example.