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Re: HF pre-selector

Posted: Sun Oct 11, 2015 3:45 pm
by NN4F
One of the best I ever found for SWL was the Global AT-2000, sadly no longer made, but you may be able to find one used
it works extremely well, or now the MFJ1046 is similar...

Paul - NN4F

Re: HF pre-selector

Posted: Mon Oct 12, 2015 2:02 pm
by CaptainNemo
By the way, "DM4070 LCR meter" could be an honest tool for the price?
(about 40-50 $ on e-bay)

Re: HF pre-selector

Posted: Sat Oct 17, 2015 11:08 am
by m0phe

Re: HF pre-selector

Posted: Wed Oct 28, 2015 5:28 pm
by ptuton
I use this one and find it excellent. Takes very little building. ... lifier.htm

With HDSDR or SDR-CONSOLE 2.x, the SDR-PLAY is now my favourite HF general coverage Rx. (One of the alternatives is an AR7030).


Re: HF pre-selector

Posted: Tue Nov 24, 2015 1:50 pm
by CaptainNemo
Thank you very much to all. I get a lot of ideas from this forum.
I'll try to build a preselector by myself.

By the way. Yesterday I was looking for info about "usb galvanic isolator" and suddenly I found a cheaper solution.
I moved SDRplay from one usb port to the near one, a lot of "vertical lines" disappeared. Great!

Re: HF pre-selector

Posted: Thu Dec 31, 2015 5:22 am
by vk3zfs
My gathering dust Yaesu FRG-7, might be put back into service as MF-HF RF pre selector, new life for a old boat anchor :)

Re: HF pre-selector

Posted: Thu Feb 22, 2018 4:05 am
Dear Captain Nemo (may I please get a ride on the Nautilus sometime ?);

A long time ago the ARRL Handbook had a 'regenerative preselector' for the HF bands. Given the era (1960s) it was based on vacuum tube technology but still .... a very interesting idea .... (as a regenerative amplifier is brought near oscillation the effective bandwidth begins to narrow very significantly)

I live only 4.3 miles from the antennas of KRLD (50 kW on 1080 kHz) and listening to anything near that portion of the AM Band can pose problems.

So far I have only been considering how to combat the tremendously strong signal coming from KRLD but I have been thinking significantly about a regenerative preselector (with variable coupling to keep signal levels under control).


Paul, WB5AGF

Re: HF pre-selector

Posted: Thu Feb 22, 2018 10:28 pm
by vk7jj
To add to Tech_Support's suggestion of using a "ham" antenna tuner, I use an old antenna tuner (commonly known as an ATU) but instead of connecting it in series with the feedline, I use it as a tuneable shunt to ground.

If the ATU is simply inline as normal then it works fine but it only passes the relatively narrow band of signals it's tuned to. The problem then is you have to retune it all the time when changing frequency or changing bands even if it's only a relatively small change.

If the ATU is connected as a tuneable shunt to ground then it gets rid of the unwanted band of signals it's tuned to and passes everything else.

The one I use is good for the amateur bands from 160m to 10m and I found in practice that as a shunt it can be adjusted to be narrower or broader so as to "notch out" the broadcast band without bothering 80m, or be more finely tuned to wipe out a particular broadcast station. It also works well in taking out bands of noise anywhere on the HF spectrum

If you have access to a spare ATU, especially an older bulky manual one whose owner has moved to nice shiny new automatic thingy, then have a go. The older and larger the ATU the better it's likely to perform. The one I'm using is an old MFJ with a roller inductor.

Put a T piece in the coax between the antenna and the RSP1a, connect the 'radio' connector of the ATU to the stem of the T piece and put a short circuit across the 'antenna' connector of the ATU, and have a play. In fact have an SDRplay :-)

Re: HF pre-selector

Posted: Fri Feb 23, 2018 7:12 am
by pb1sam
Thank you for the good idea.
Maybe I can use this also on a Flora & Fauna activation.
A friend and me running two stations, one on 20m and one on 40m.
Because we are a couple of meters apart from each other,
we have interference of each other.
Maybe with this idea we can eliminate each others signals. :)

Re: HF pre-selector

Posted: Wed Feb 28, 2018 5:30 am
by vk7jj
As a result of an email from a member I thought this might help illustrate the notching ability of an old ATU.

Did an experiment with the setup as illustrated below.

1) cheap eBay noise generator with it's output loosely coupled to an RSP1A input T connector
2) pretty ordinary low Q Kenwood old mobile ATU that does 80m to 10m
3) ran the RSP1A from SDR-Console v3 with 8MHz bandwidth and fixed AGC at 35dB. SDR-Console made it easier to adjust waterfall frequency and dynamic range.

Cheers and 73,


PS. Forgive the cheap and nasty camera shots of the screen, didn't have time to do proper screen dumps and move them to the computer inside.
PPS. Computer hardware is an ageing 27" iMac running Windows 10 in VMWare Fusion allocated only 2MB RAM and using 2 cores

Picture1 shows 4.6MHz to 9.4MHz
Slid the ATU from 7.1MHz down to 5MHz as you can see by the moving blue notch
The notch depth is roughly 26dB.

46to94.jpg (68.49 KiB) Viewed 6194 times

Picture2 shows 1.4MHz to 4.3MHz
Shows the notch at 2.6MHz
The notch depth is roughly 33dB.

1to4.jpg (71.44 KiB) Viewed 6194 times

RF white noise generator with output loosely coupled by wrapping a few turns around an alligator clip sticking out of the antenna side of the T
The ATU 'radio' connector is on the other bar of the T
You can see the short circuited UHF connector on the 'antenna' input of the ATU

setup.jpg (85.57 KiB) Viewed 6194 times