LF Stuff
Frank Gentges KØBRA
fgentges@mindspring.com
Sat, 01 Sep 2001 17:28:07 -0400
Brooke,
Thanks for the comments and contribution.
At 06:30 PM 8/31/01 -0700, Brooke Clarke wrote:
>Hello:
>
>I have been interested in LF for many years and for a few of them built
>and maintained a beacon on 175 kHz "LAH".
>
>Active Antenna
>
>The QST article brings up some comments:
> * The heat sink and mechanical packaging leave something to be desired.
Yes, I was surprised how few commercial heat sinks are made for the TO-5
transistor case. The few existing ones are way short of the delta T
requirement. We also had the problem of the JFET gate being connected to
the case so we had to position the heat sink so we would not get
oscillation from the output. Now if I could only find the CP-666
performance in a TO-220 case. (I already asked)
If you want, you can buy the same chip in a surface mount and make up a
better heat sink arrangement than we used. We felt surface mount and the
special PC board would be overcomplicate the design and make it too
difficult for the average Radio Amateur. We would be interested in any
work you do with this.
> * I would suggest using an outdoor single switch electrical box.
I used one of these for early prototypes. When extended on a long pole in
the winds, the cross section was too much, at least for the Outer
Bankjs. The PVC pipe was a real improvement in this.
Dymek used a bunch of transistors in their circuit and required the
additional space that the box would provide. Our test of the Dymek DA-100
showed it had quite uneven frequency response, overload and intermodulation
problems so that steered us away from their level of complexify.
Once you put gain in the active antenna, if it overloads, you cannot back
out the trash it generates. The additional gain should be added in the
shack where it can be switched in and our and filtering can be added as
needed. We tried to keep the outdoor part simple.
> * This was done for the Dymek DA-100 active whip and has lasted about
> 40 years for me.
The PVC pipe seems to seal quite well for us even in the salt fog of the
Outer Banks.
> * In one case where lighting was a few miles away the input transistor
> was smoked and replaced. I feel that the current design will fail much
> sooner. Some stronger lighting protection may be called for?
We put a neon bulb plus the 68 pFd capacitor in the input circuit to limit
the energy that could get to the gate. We felt this was superior to the
input protection in most other designs. We looked at the spark gap tubes
but found the NE-2 from Radio Shack did better at lower voltages plus being
more readily available. On the other hand, the NE-2 is prone to fail with
nearby strikes. We chose the simplicity and availably of the neon.
> * An alternate packaging that was as light as possible and with a
> small drag coefficient so that it could be mounted on a commercial pain
> roller extension pole would also be good as an option for portable use,
> but the outdoor electrical box is in my opinion about optimum for a fixed
> station location like mine.
We had an early prototype that ran on 12 volts at 28 ma. in a piece of
one-half inch PVC pipe. We could not achieve the intermodulation
performance we needed for transatlantic signal strengths so we went up to
24 volts and 53 ma. bias current. The heat problem pretty much dictated
the larger package.
We use 24 foot paint roller poles on our expeditions to the Outer
Banks. The QST design works well with those poles for portable ops.
> * Somewhere, either in the QST article or on your web page I read that
> a filter needed to be added to the active antenna in order to get the
> noise floor down to where it should be. This points out the need for
> filtering as close to the antenna as possible. Adding a low pass filter
> in the power supply box that could be switched in would be a useful thing.
Stay tuned. We are working on an LF upconverter and we have a nice low
pass filter with that. That same LPF could be put in front of the receiver
if it cannot deal with the strength of the AM broadcast signals. If you
choose you can incorporate it into the power supply.
> * Also it was mentioned that using a multicoupler was very handy, yet
> there didn't seem to be any information on how to make a good one. This
> too could be part of the power supply box for the active antenna.
We wound a 4:1 splitter transformer that was not a hybrid that worked
well. We also use multiple frequency selective filters in parallel so you
can listen to 60 kHz, 136 kHz, 160-190 kHz, AM broadcast and HF off of one
antenna.and hope to have something published on this before too long. QST
gave us 5 pages, we used 7 and still a lot of nifty stuff could not be
included.
> * There might be options for the transformer such as:
> * what turns would be good for someone in a very quiet area (I'm
> in the boonies and this would apply to me)
The transformer design is pretty much dictated by the bias current rather
than the signal strength.
> *
> * What Litz wire may give better results for the transformer
Losses here are not significant and litz wire would make only an
infinitesimal change.
> * There are a number of people interested in Time & Frequency who are
> looking for a good antenna for 60 kHz work.
> * An option to optimize this antenna for that application would be
> most welcome.
We have built some bandpass filters that do not suck out signals at other
frequencies. It uses series resonant tank circuits rather than parallel
resonant tank circuits. We hope to publish some material on this. In the
meantime, just add a 60 kHz bandpass filter.
>
>Web Page Problems
>
>
>The link
><http://www.hamwic.co.uk/radio/vis.html>http://www.hamwic.co.uk/radio/vis.html
>is broken, it appears on your page:
><http://www.amrad.org/projects/lf/>http://www.amrad.org/projects/lf/
We will remove that link on the next update.
>
>
>
>
>Field Strength Meter
>
>
>For a modification of the Army ME-61 designed for Lowfer work see see my
>web page:
><http://www.pacificsites.com/~brooke/ME61.shtml>http://www.pacificsites.com/~brooke/ME61.shtml
Nice mod. Looks like it could be built from scratch as no parts are that
special.
Alternatively, you can use an AC VTVM like the HP-400 on the output of the
AMRAD active antenna and be able to calculate the actual field strength of
the signal from Figure 1. From that you can derive the radiated power and
the true antenna efficiency.
>
>High Power Digital TV modulator and Linear Amplifier
>
>
>Are these misplaced on your web page:
><http://www.amrad.org/projects/lf/>http://www.amrad.org/projects/lf/ or
>is there some connection to LF?
You are right about the connection to LF. We added it as a favor to some
people that were interested in our work on this.
>
>Spectrum Analyzer plot 50 to 150 kHz at my QTH 39.19N 123.164W
>
>
>150.tif attached made at about 01:00 UTC 1 Sep 21001 using an Agilent 4395A.
>You can see that LORAN-C eats up 80 to 120 kHz. You can easily hear the
>LORAN-C pulses on 140 kHz.
That is the same problem we had on the Outer Banks efforts to hear the
European Radio Amateurs. We have been working on a number of prototypes to
solve this. Our active antenna plus a 136 kHz band pass filter plus an
RX320 or a Cubic R-3030 is pretty much clean of Loran-C spectral lines. It
was not trivial achieving this as I am sure you can appreciate.
>
>That is to say that trying to receive signals in the 135 to 138 kHz band
>will be difficult without a LORAN-C blanker.
>
>Just some thoughts,
>
>Brooke Clarke, N6GCE
><http://www.pacificsites.com/~brooke/>http://www.pacificsites.com/~brooke/
>
>
>
>
>
>
>
You would make a good AMRADer to work on our LF projects.
73,
Frank
Frank Gentges KØBRA
<fgentges@mindspring.com>
LF web site at <http://www.amrad.org/projects/lf>