On Jul 26, 6:28*am, Wimpie wrote:
On 26 jul, 03:23, Ross Archer wrote:



I'd like to try building an *untuned* loop amplifier covering HF and
wonder if anyone has seen links on this.

The thinking so far is using a pair of 2n2219a bipolar transistors
running a few hundred milliamps, with their collectors driving
opposite ends of an RF output transformer, and a reasonably large loop
(say 1 meter square or more).

I'd like the output to be "hot" enough to compensate for a losses from
a nice long run of cheap cable-TV type coax cable (through which power
will be fed up to the amp). *But not so "hot" as to create distortion..

Anyone here have experience with this sort of project? *If so, is the
2n2219a a stupid choice? *Would I be better with JFETs, and would a
single-ended amplifier work almost as well as a balanced amp? *(My
thinking is the balanced amp would potentially be quieter.)

It would be fun to try this, and my junk box is overflowing with
parts, so why not?

-- ross

Hello Ross,

I did some experiments, with same type of transistors. My loop was
made from chicken mesh to reduce the impedance in the high part of HF.
Low impedance reduces the contribution from the base current noise
from the transistors.

I had to do some tweaking to avoid parasitic oscillation. I also had
intermodulation distortion from VHF that was noticeable at around 5
MHz. The reason was the increased sensitivity of the loop around the
VHF broadcast band. *Adding RC section parallel to the loop did reduce
this significantly (below noise level at HF).

Other issue was intermodulation in my receiver (I checked this by
adding a tunable filter between the amplifier and the receiver).
Reducing the gain by adding some emitter resistance did help.

In the end I changed the loop into a loop with natural balun (same
principle as used in small loops for EMC purpose, the have a cut in
the middle [opposite to the feed point]). About 10 m low loss coaxial
cable (with common mode ferrites) *is between the loop and a simple LC
parallel circuit with selectable tabs to have some preselection and
matching.

I also built a 2 stage non-balanced amplifier with feedback via the
emitter of the 2n2219 input transistor (all inside the antenna loop
itself, fed over the coaxial cable). Using 2n2219 with 2N3906 resulted
in intermodulation.

Changing to BFR91A (input stage) and BF979 (buffer, is PNP, I used two
devices in parallel) gave a significant reduction of intermodulation
(because of higher open loop gain). *So if you want to use 2n2219A as
a single stage, a balanced amplifier will definitely better then an
unbalanced stage.

I live in an almost flat area (valid for most parts of the
Netherlands) with many signals (and noise) present. So I didn't need
additional amplification for HF (based on chicken mess loop with 1m
diameter). *When you live in a remote area you probably need some
gain.

Best regards,

Wim
PA3DJSwww.tetech.nl
please remove the obvious character combination in case of PM.

Very interesting, and thank you for the feedback.

Do I understand that the "natural balun" is the 10m length of coax
with ferrites to choke off any unbalanced currents? Is there something
important about the "break" in the loop opposite the feedpoint, do you
think?

I'm not set on any particular transistor, or even technology. NPN,
PNP, JFET, MOSFET, IC, whatever works best.
My junkbox has the following in serious numbers:
BF998
2n4416a
J310
MPF102
2n3819
PN2222a
MPSH10
MPSA18
2n3904
2n3906
and a few:
NTE10
2n2219a
2n2218

I had a few 2n5109s, which might be even better, but can't find them.

The real reason I'm starting this is I was re-organizing and found my
parts stash, and figured it's time to build stuff again

-- ross