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 Post subject: Modifying an old receiver to double as a wave trap.
PostPosted: Apr Thu 20, 2017 3:30 pm 
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I've got a very hot MW station next to my house. I've also got an old Kenwood KR-2200 that has damage to the concentric tone/volume control. I was thinking of shunting the Kenwood's tuner to ground and using it as a wave trap. The MW station is overloading the front end of my SW radio and really getting me down. I want to hack the kenwood in such a way as to remove the ground connection and have the hack be more or less transparent. Has anybody here thought about something similar?

-DanBL


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 Post subject: Re: Modifying an old receiver to double as a wave trap.
PostPosted: Apr Thu 20, 2017 4:12 pm 
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Well I suppose you could hack into a radio and use a tuned circuit there to block a BCB signal, but that seems to me to be a rather bulky wavetrap. I would use something like a smaller hi Q coil and tuning cap for that purpose. You would probably want to put it in a metal box for shielding and a very short or shielded connection between the trap and the receiver. Does your SW radio have coaxial input? As you would get the most rejection using a shielded system and probably some sort of filter on the AC line going in, too, if you are really in close proximity to the trans.


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 Post subject: Re: Modifying an old receiver to double as a wave trap.
PostPosted: Apr Thu 20, 2017 5:06 pm 
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wazz,

Thanks for your reply. I was thinking of building a setup identical to the one you described except that I have a TRS antenna input on the SW (Tecsun PL-880) and I use it with a battery. I was just thinking about what I had on hand to use as a wave trap. I've got some chores I need to ignore today so I'll do some tinkering on the Kenwood. I stand a good chance of getting nowhere except to maybe learn something. BTW, I've been doing a lot of reading on Antique Radios and want to thank the folks that administer and contribute: thank you.


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 Post subject: Re: Modifying an old receiver to double as a wave trap.
PostPosted: Apr Thu 20, 2017 5:21 pm 
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Location: Central Pennsylvania
Your wave trap needs to be set once to a single frequency in the BCB. The Kenwood receiver front end tunes continuously across the medium and short wave bands.

Why not use a single inductor and required capacitors to make a single frequency wave trap?

If you choose the right toroid core I am sure you can get higher Q and better performance with a single frequency wave trap.


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 Post subject: Re: Modifying an old receiver to double as a wave trap.
PostPosted: Apr Thu 20, 2017 6:19 pm 
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I once made a wave trap out of an old 2-transistor radio. It was a Mickey Mouse radio, and the ears were tuning and volume.
This was many decades ago, and it had no particular value at the time.

It was truly a mouse trap.....

Bob

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 Post subject: Re: Modifying an old receiver to double as a wave trap.
PostPosted: Apr Thu 20, 2017 8:09 pm 
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Try this;


Attachments:
Antenna Wavetrap.jpg
Antenna Wavetrap.jpg [ 10.41 KiB | Viewed 373 times ]

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 Post subject: Re: Modifying an old receiver to double as a wave trap.
PostPosted: Apr Thu 20, 2017 8:15 pm 
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Location: Albion, CA, USA
A friend of mine is suffering from the same problem. We started with this:

http://www.dxing.com/tnotes/tnote06.pdf

Using the wave trap, we see a 45dB reduction.

We have ordered the parts for this:

http://vk3il.net/projects/broadcast-band-filter/

Because he wants a filter that he doesn't have to switch out during TX.

Good luck!


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 Post subject: Re: Modifying an old receiver to double as a wave trap.
PostPosted: Apr Thu 20, 2017 8:24 pm 
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A wave trap works the best when the capacitor is in parallel with the coil creating a resonant circuit.
Attachment:
WaveTrap.jpg
WaveTrap.jpg [ 16.58 KiB | Viewed 369 times ]


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 Post subject: Re: Modifying an old receiver to double as a wave trap.
PostPosted: Apr Thu 20, 2017 8:55 pm 
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For that matter, you could use the tuning cap and antenna coil from a junked transistor radio. But I still think that it would be most effective if it were shielded from the input to the trap to the ant. input of the receiver. In a very intense field. I feel that having the trap out in the open and much of a connection between it and the rec. would allow some leakage and might matter or not, in ultimate rejection of the BC sig. Using the loopstick of a trans set and the tuning cap section that is attached to it in the set, you already know it will tune to the sig. of the BC station, no further design work there. Then carefully tune for greatest reduction in interference.


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 Post subject: Re: Modifying an old receiver to double as a wave trap.
PostPosted: Apr Fri 21, 2017 2:31 am 
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fifties wrote:
Try this;
This is a series circuit, lowest impedance where tuned... So, you would try to tune it to the SW station you are trying to receive (I did not estimate the range it would tune to, but it does not look SW to me.) Anyway, might be hard to use and no guarantee that it would allow the desired station to get though the interference. IMHO, better to use a parallel tuned circuit, in the antenna line, tuned to the interfering local BCB station to block as much signal as possible.
Cheers,
Roger

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 Post subject: Re: Modifying an old receiver to double as a wave trap.
PostPosted: Apr Fri 21, 2017 7:07 am 
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I would then suggest that he obtain a coil and tuning capacitor, and experiment with it in each configuration, series and parallel, and of course use that which works best.

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 Post subject: Re: Modifying an old receiver to double as a wave trap.
PostPosted: Apr Fri 21, 2017 2:09 pm 
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Yes, a series tuned circuit or some kind of selective "antenna tuner" could work to reject out of band interference, but it would have to be constantly retuned/repeaked for whatever station you are tuned to. A parallel tuned trap would be only tuned to the freq. of the BCB station, set and forget. Ameco(and others) used to make preselector/preamps for SW radio use. Had gain for those that were weak on the higher bands and the tuned circuit helped get rid of images in single conversion sets, as well as keeping out stuff well out of range. Hams on 160M bands used to have trouble with BC stations getting into their desired freqs above 1.8 mHz. Maybe still do. So there were designs for high pass filters to block out stuff below 1.8 mHz. That would require no tuning but is obviously a more complicated device to build.


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 Post subject: Re: Modifying an old receiver to double as a wave trap.
PostPosted: Apr Fri 21, 2017 2:13 pm 
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Example: http://www.ebay.com/itm/AMECO-PREAMPLIF ... SwtZJY9YyO
They made both nuvistor and solid state FET version, both for SW and with the bypass relay inside for transmitting for hams.


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 Post subject: Re: Modifying an old receiver to double as a wave trap.
PostPosted: Apr Fri 21, 2017 6:13 pm 
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wazz wrote:
Yes, a series tuned circuit or some kind of selective "antenna tuner" could work to reject out of band interference, but it would have to be constantly retuned/repeaked for whatever station you are tuned to. A parallel tuned trap would be only tuned to the freq. of the BCB station, set and forget.

Couldn't a series tuned circuit be used as a shunt from antenna input to ground?

Wouldn't this be just as effective as a parallel tuned circuit in series with the antenna?


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 Post subject: Re: Modifying an old receiver to double as a wave trap.
PostPosted: Apr Fri 21, 2017 7:57 pm 
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Joined: Dec Mon 20, 2010 5:14 pm
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Location: Central Pennsylvania
Yes a series resonant LC circuit resonant at the interfering Broadcast stations frequency would work and i would try that first.

In theory a series resonant circuit to ground and a parallel resonant circuit in series with the antenna lead will both work.

In practice generally one or the other will be more effective depending upon the receiver input impedance, the antenna impedance, the length and the impedance of the transmission line connecting the antenna and the receiver, the specific inductance and capacitance values used in the trap, the amount and location of stray or parasitic capacitance and or inductance, etc. etc.

interruption


I am at work and a junior engineer just asked me to sign his Professional Engineering exam application whihc leads me to consider a quantitative answer to the "How should i connect the wave trap?" question would be a good PE exam question.

That is why the easiest thing to do is just experiment. I suggest trying the shunt connected series resonant trap first. Initial attempts should be made with alligator leads for connection but when you find something that works for you neaten everything up.


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 Post subject: Re: Modifying an old receiver to double as a wave trap.
PostPosted: Apr Sun 23, 2017 7:06 pm 
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Try an antenna tuner like a 'L' network for simplicity.
Image
Image compliments of K6JCA.

With a tapped coil and a variable cap switched to either side you can have a high pass or low pass filter. Adjust tuner for max signal strength of the shortwave station you are listening to.
And you don't need the coil shorting switch unless you want it.

Here's another version.
Image

Here's one I made for a little low power ham radio I built.
Image
Image

The coil is 1" dia and 32 turns but I don't remember what the turns per inch were. Maybe 12 T/I?
The variable cap is for a transistor radio. Both sections ganged together for about 300 pf total.


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