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HF/MW/LW Equipment - For general and technical discussion of all receivers which cover the HF bands. For HF tranceiver discussion please use the Amateur Radio Equipment forum above.

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Old 04-11-2017, 9:59 PM
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Default A new preselector from the UK

The new Cross Country Wireless HF Preselector | The SWLing Post

Let's talk briefly about a preselector. It acts as a tunable 'gate' that rejects out of band signals and other spurs, such as FM stations showing up on HF. These are very useful for older single conversion desktops for this very reason.

Broadbanded SDRs such as the RTL-SDR and even the SDRPlay would benefit from such a device, especially when using antennas of any size near an urban area where MW and FM stations abound, often running with a bunch of power. Their front ends are not designed to work well in such environments.

There are 2 kinds of preselectors - passive and active.

Passive units like the one above, and the 2 sold by MFJ, are preferable in high RF density areas since they don't have an amplifier.

Palomar used to sell the P-305 (I think they also made a P-308) preselector that had a tunable amp with about 20db of gain - this is an example of an active preselector. The P-305 had a gain control that allowed you to drop the gain if needed. These are great in remote or less developed areas, where the additional gain would cause less problems with overloading issues.

There are schematics on the web for these units, but construction can be a bit of a challenge, particularly if you're unaccustomed to winding toroids. Proper bonding and shielding is an absolute must.

If there are other commercially made preselectors that you know about, feel free to post the URL here.

Mike
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Old 04-15-2017, 4:42 PM
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I haven't tried it but it would be easy to implement - I (and many others?) have an FRG7 that has a front end preselector that requires no power - a small capacitor feeds the RF amplifier. It should be possible to add another capacitor before C101 to a socket on the rear apron to feed any other receiver.
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Old 04-15-2017, 5:30 PM
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It might help some, Martin, if you

a. Tell us what this is schematic is from (looks like an active amp circuit)...and...
b. Precisely what 'would be easy to implement', and why...it sounds like you are tapping the preselector on the FRG7 for some purpose...

This post resembles some half-considered thought (probably a good one), but we're not Vulcans here...(heh)

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Old 04-15-2017, 6:11 PM
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My apologies, Mike, it probably is a bit confusing.

Well, as I mentioned in the text - admittedly it was pretty brief - it's the preselector of a Yaesu FRG-7 receiver, very popular since it first came out in the 70's - the first domestically available communications receiver to use the Wadley loop - apart from the Barlow-Wadley XCR-30 which was made in South Africa and not in great numbers. Your opening post was about a preselector now available from the UK and it occured to me that anyone with an FRG-7 - and there must be many - could add a simple modification to their radio to make use of the internal preselector without much effort.

To explain the schematic diagram - top left is the antenna input socket J1 which is connected to a switchable attenuator comprising all those resistors R1 to R6 and the front panel switch S1. Then comes the preselector itself - the frequency band selector switch S2a, S2b and S2c. The tuned circuit inductances T101 to T104 are tuned to frequency by the front panel control capacitor VC1. So the selected peaked frequency is passed to the RF amplifier Q101 through C101, 0.01uF. That's as far as we need to go, we have found all the necessary components of the preselector.

Now my suggestion, which I'll test hopefully today, involves adding another capacitor of 0.01uF at the junction of C101 and the terminal on the PC board which goes to the wiper of S2c and leading it to a new socket on the rear panel. This socket can then be connected to the antenna socket of your SDR with the existing HF antenna now connected to the socket J1 on the rear of the FRG-7.

So now you have a preselector for your SDR and it's almost free! I'll make a trial version and see if it works - one problem I can see is an impedance mismatch but one test is worth a thousand opinions!

Thanks for your comments.
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Old 04-15-2017, 7:49 PM
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Unfortunately the one test is a failure! As I surmised, the low impedance of the SDR kills the 'gain' effect of the tuned circuits and in fact sucks out the signal. It does work if you go further down the FRG7 receiver to a low impedance point but that needs power on which rather spoils the whole idea of 'no power'. Oh well, back to the drawing board! Maybe a matching transformer....watch this space....
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Old 04-17-2017, 9:07 PM
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A few radios of that era had built in preselectors, the Drake SSR1 being an example.

The one in the FRG-7 works really well with that radio.

I've heard good and bad about external preselectors. One would think they get the job done a lot better than antenna tuners.
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Old 04-17-2017, 9:34 PM
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Don't mix up preselectors and tuners. In very simple terms, a transmatch (which is a more accurate name than an antenna tuner) attempts to insure that there is a max current flow between the antenna and feedline and the receiver; a preselector - active or passive - is a tunable 'filter' that allows only a select group of frequencies to be passed while severely attenuating others. While some matching does take place, that's not a preselector's main function. Transmatches can't offer nearly the amount of attenuation that a preselector can..

A true antenna tuner is usually placed at the feedpoint of the antenna, not at the station position. This insures that the tuner is working primarily on the antenna itself, and is not trying to match both the antenna and feedline. Such devices are pretty expensive..

All 3 have their place, to be sure, but it's important to understand the basic differences. Mike
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Old 04-18-2017, 5:00 AM
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Quote:
A true antenna tuner is usually placed at the feedpoint of the antenna, not at the station position. This insures that the tuner is working primarily on the antenna itself, and is not trying to match both the antenna and feedline.
Definitely, and this is something that seems to escape a lot of people. A 'tuner' works best when it is fed with a bit of wire directly from the antenna - but as soon as to connect the bit of wire that you have called an antenna to a coax cable via a transformer down to your receiver and then try to 'tune' it with a 'tuner' at the receiver end it doesn't work as expected - in fact you have made your 'tuner' into a preselector. As Mike has said, the best place for a tuner is at the base of the antenna - then you can feed the coaxial cable to your receiver. Unfortunately mostly we want to feed our receivers with coax cable to eliminate the local interference that an unscreened downlead will pick up. There are some very nice 'auto-tuners' for the ham guys that you can sit at the base of the antenna if it's a vertical or at the the open wire feeder point for a dipole but they require transmitter power for them to find the 'in tune' point - when they get to design one that works remotely with a receiver and knows what frequency I'm listening to, I'll be first in line. It could be done with synchro motors to turn the switches and capacitors or some form of detecting the right impedance but ....
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