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Old 16th Feb 2020, 8:33 pm   #1
lloydwells
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Default Aerial blocking circuit for TRFs anybody any ideas

Hi all I live in Dumfries I have a fairly large collection of TRF sets the problem I have is the Talk sport have a seemingly hugely powerful MW transmitter five miles up the road I'm in need of a circuit for an aerial blocker to filter this football based menace out if anyone can help.
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Old 16th Feb 2020, 9:00 pm   #2
G6Tanuki
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Default Re: Aerial blocking circuit for TRFs anybody any ideas

I'd suggest a parallel-resonant circuit in the antenna-lead.

You can achieve this using the ferrite-rod/coil and tuning-capacitor from a scrap transistor-radio.

[Something similar was commonly fitted in 1940s/1950s radios, but resonant at the radio's intermediate frequency, to stop interference from aviation navigation-beacons operating in the 300-500KHz range]
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Old 16th Feb 2020, 10:12 pm   #3
Ed_Dinning
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Default Re: Aerial blocking circuit for TRFs anybody any ideas

Hi lloyd, the period look could be achieved by means of a loop/ frame aerial with a tuning cap. There have been threads on this in the forum previously.

Ed
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Old 17th Feb 2020, 10:32 am   #4
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Default Re: Aerial blocking circuit for TRFs anybody any ideas

A loop or frame aerial may pick up more signal than it will reject.
Twould be better to use a small 'square' coil or ferrite ring.
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Old 17th Feb 2020, 12:36 pm   #5
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Default Re: Aerial blocking circuit for TRFs anybody any ideas

Not being near a powerful MW Transmitter - the nearest is Absolute Radio's Beckley relay -I don't have that problem, but I'd have thought a screened can tuned to Talk Talk's local Frequency would have been a better choice - as Trevor suggests, use a small 'square' screened coil & parallel capacitor., wired as G6Tanuki suggests.
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Old 17th Feb 2020, 2:18 pm   #6
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Default Re: Aerial blocking circuit for TRFs anybody any ideas

What you need is what is known as a 'rejector circuit' in the aerial down-lead, fitted close to the radio. That cct. is nothing more than a coil in parallel with a capacitor - ideally a variable one, initially, until the precise value can be found by experiment and consequent measurement and a fixed vale of capacitor fitted instead. Having said that, if the variable is left in place, no harm will be done.

The resonant freq. of the L/C cct. should equal the freq. you wish to reject. Such a parallel tuned cct. presents a high impedance to that freq. It will be desirable to aim at a fairly high Q, so the L/C ratio needs to be at least 100. Use an air-spaced coil with a large diameter to length ratio. A coil diameter of about 3" would be a good start.
If you have access to a Q-meter, then the whole job will be a lot easier.

Al.
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Old 17th Feb 2020, 4:39 pm   #7
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Default Re: Aerial blocking circuit for TRFs anybody any ideas

Hi LLoyd,

You may also try a series-tuned circuit across the 'aerial' & 'earth' terminals.

The inductor and capacitor may be identical to the ones used in the parallel-tuned as described by Al.

Regards,

Nandu.
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Old 18th Feb 2020, 1:57 am   #8
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Default Re: Aerial blocking circuit for TRFs anybody any ideas

Quote:
Originally Posted by vu2nan-nandu View Post
You may also try a series-tuned circuit across the 'aerial' & 'earth' terminals.
The L/C cct. in that config. is usually known as an 'acceptor cct'. That's because at resonance, it's a low-impedance - so it 'accepts' the current.

Now in theory, an arrangement could be made to have both: acceptor cct. and rejector cct. The problem with that is that the variable caps. will be very interactive, making tuning really tricky - especially if the Q of each is high - which will be required.
Also with the acceptor cct., used with a 'longwire' and an earth connection, that earth connection does need to be a really low Z. Otherwise the high-Q of the acceptor cct. will be seriously compromised.

A much more elegant way of solving this problem would be to devise a way of introducing a 180 phase-shift signal at the required freq., with it's amplitude adjustable, and thus eliminate the offending signal by phase-cancellation. How to physically do that? I don't know! Perhaps someone else here just might . . . .
At the back of my memory, I think that approach was tried many, many decades ago under the name of the Jones Circuit. I have the vague notion that it have featured in the RSGB magazine / Technical Topics during the late / mid-1960's.

Al.
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Old 18th Feb 2020, 1:56 pm   #9
Bazz4CQJ
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Default Re: Aerial blocking circuit for TRFs anybody any ideas

I wonder if a magnetic loop antenna might be worth considering? Their most appealing feature is their immunity to electronic noise, but the they are directional with significant nulls which could be directed at the interfering station.

You should find 2 or 3 quite lengthy old threads on this forum, and tons of references via Google about magloops. The commercial magloop made by Wellbrook sells at over 300, and those who have one swear by them. Alternatively, its clone, the "Wellgood", can be built for next to nothing (I built one). One or two people on the forum are pretty keen on them.

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Old 18th Feb 2020, 2:02 pm   #10
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Default Re: Aerial blocking circuit for TRFs anybody any ideas

Hi Al,

Connected across the 'aerial' & 'earth' terminals, the series-tuned circuit also rejects the offending signal by bypassing it to earth.

Regards,

Nandu.
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Old 21st Feb 2020, 9:02 pm   #11
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Arrow Re: Aerial blocking circuit for TRFs anybody any ideas

Quote:
Originally Posted by vu2nan-nandu View Post
Hi Al,
Connected across the 'aerial' & 'earth' terminals, the series-tuned circuit also rejects the offending signal by bypassing it to earth.
Which is what I said in post #8 - and also named that cct.

Al.
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Old 21st Feb 2020, 10:04 pm   #12
TrevorG3VLF
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Default Re: Aerial blocking circuit for TRFs anybody any ideas

A crystal has an equivalent circuit which can accept and reject at two close frequencies. I am sure someone cleverer than me could design a circuit with one inductance and two capacitors to do the same job on MW.
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Old 25th Feb 2020, 12:59 am   #13
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Default Re: Aerial blocking circuit for TRFs anybody any ideas

Possibly: one L and two Cs. However, for a deep, sharp null at the required freq., the Q of that coil will need to be quite high - probably in excess of 300 - which could be quite difficult to obtain at MW freqs.

Al.
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Old 6th Mar 2020, 3:55 pm   #14
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Default Re: Aerial blocking circuit for TRFs anybody any ideas

Llloyd- simpler solution comes to mind, from an idea from a cousin who was a TV engineer in Glasgow. Hill head tube station had a reputation for causing RFI, and his solution was to fit a mesh screen ( Technical name for a length of chicken/or similar wire, shorted to earth), behind the aerial). Might be a cheap & easy fix.
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Old 6th Mar 2020, 6:15 pm   #15
PJL
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Default Re: Aerial blocking circuit for TRFs anybody any ideas

A 1925 Marconiphone 'rejectostat' can be seen in a picture half way through this thread https://www.vintage-radio.net/forum/...d.php?t=146641

As described by Al, it is simply a tuned circuit with a few taps on the coil to cover multiple ranges, the whole thing enclosed in a metal case. Can't say if it works because I still haven't finished the plugin coils (got as far as getting the primaries working and tuning in the correct range).
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