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.

I imagine intermodulation would be a serious concern in The
Netherlands. How does your most recent setup work?

Years ago I had a Wellbrook ALA1530, and it excelled at picking out HF
signals without picking up nearby electrical noise, until, attaching
the wrong coax connector to a 50 watt transmitter, I accidentally
tried it out as a transmitting antenna. Needless to say, the
results were unpleasant.

Short of buying a new loop, I thought it would be fun to try this. It
sounds like you found some configurations that were pretty resistant
to intermod.

The BF979 is surprising. PNP transistor that has an Ft of over 1300
Mhz? I didn't even know that was possible.

I built the 2n2219a as a single-ended amp to evaluate, with 142mA of
current standing and a VCE of 6.8 volts with Vcc=13.1v. That's bad
practice since VCE 1/2 VCC, but that's just how it turned out using
standard parts. So far it hasn't melted.

Anyways, I haven't detected any intermod with about 10 feet of wire.
The transistor runs damn hot though

Thanks for all the ideas! Have a great week!

-- ross






My goal is to find something relatively cheap to build and flexible
enough to work in weak HF signal areas that also doesn't overload in
strong signal areas. Kind of a VW bug version of an ALA330 for the
budget-minded.