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Old 5th Nov 2020, 11:42 am   #1
ORAWA01
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Default Advice on the LOW PASS Filters

Hi All

Just wondering if LOW PASS filters for blocking MW signals appearing on SW or LW bands are worthwhile investment.

When I RXing 4-5 Mhz the tropical bands, MW signals appear all over the place when using larger antennas and amplified antennas at nights.

And the strong MW signals also appear on LW bands too.

Would it be effective to use the LOW Pass filters commercially available? Are they not supposed to degrade the signals that meant to be received in the bands? What are your thoughts and experiences on this?

Thanks
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Old 5th Nov 2020, 1:28 pm   #2
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Arrow Re: Advice on the LOW PASS Filters

Quote:
Originally Posted by ORAWA01 View Post
Just wondering if LOW PASS filters for blocking MW signals appearing on SW or LW bands are worthwhile investment.
I may have misunderstood the above, but if a low-pass filter is installed to block MW signals then all signals above MW frequencies will be blocked also, i.e. all signals on the SW bands. However, since LW signal are lower in frequency that MW signals, they will will not be so blocked.

However, judging from what you have written, it does sound to me that the fitting of such a filter, be that low-pass, high-pass or even band-pass is simply tackling your problem the wrong way. It does sound as though you have an overload of signal strengths reaching your receiver producing severe cross-modulation. There is a strong possibility that there is a fault in your receiver that is allowing strong off-tune signals to cause such cross modulation: I would investigate that path first.
Failing that, the fitment of an AMU / ATU * between aerial and receiver would be worth a try or even a variable resistive attenuator should help.
There is also the possibility that in your locality there may be a transmitter producing high field strengths driving the 'front end' of your receiver into non-linearity, in which case, the fitment of that ATU / AMU, perhaps plus an attenuator, should help.

A further thought: You also wrote "using larger antennas and amplified antennas at nights". That is really not helping matters! Your aerial is probably picking up quite adequate signal strengths: the amplifier is probably getting driven into non-linearity. Remove that amplifier and see if doing that helps.

*Aerial matching unit / Aerial tuning unit

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Last edited by Skywave; 5th Nov 2020 at 1:33 pm. Reason: Add last para.
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Old 5th Nov 2020, 1:35 pm   #3
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Default Re: Advice on the LOW PASS Filters

You'd need a _high_ pass filter, with a turnover-point probably around 2MHz, to do what you want.

This was one of the benefits of an outboard 'preselector' that was a popular add-on to receivers in the 50s and 60s - they weren't always needed for extra gain, sometimes improved selectivity/image-rejection was more important.

What is/are the radio(s) you are using? Proper 'communications' receivers with several signal-frequency tuned-circuits should cope with the output from a decent longwire with ease.

[Though there are some examples of 'professional' radios - the receiver of the Pye C12 military transceiver, for example - where "MW Breakthrough" can be a real issue when working with long antennas]
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Old 5th Nov 2020, 2:14 pm   #4
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Default Re: Advice on the LOW PASS Filters

Yes, my misunderstanding on the concept and terminology. Apologies.
Thanks for pointing it out.

I will need a High Pass filter to attenuate the strong MW signals appearing on the SW bands.

It should suppress any signals below 1.75 Mhz, but freely pass anything above that. That is what I was looking for. I was wondering how effective these device would be.

My receivers can be anything vintage such as FRG-7, 9R-59D to more modern ones like Tecsun S-2000 and IC-R75.

When the unwanted MW signals appear on the SW bands, it is usually when I am using the long wire in the garden at nights. I also use preselector and ATu, but these are not able to block out the MW breakthrough completely. And it happens to all the receivers I have tried, so it seems not fault with the receivers.

Yes, it makes sense to say that I should not be using unnecessarily long and large antennas on HF. But if I am trying to copy the DX signals which are weak and buried in the noise, then in my BCL experience, the larger antenna used to give more chance to copy, but it has this drawbacks of cross modulation and overloading.

Using smaller HF antennas would have no problem with these situation of cross modulation and overloading, but then there is less chance of hearing the weak DX signals from low powered South America or Far East signals at nights, although it would be perfectly good enough to listen to the strong and more closer stations such as Radio Romania, or the mega powered Chinese broadcast signals.
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Old 5th Nov 2020, 5:46 pm   #5
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Default Re: Advice on the LOW PASS Filters

You could always build a multiplle-tuned-circuit tunable filter to add pre-receiver selectivity - this will be good at keeping-out _all_ unwanted signals whether they're above or below the frequency you happen to be wanting to receive.

It does mean you have another knob-to-twiddle as you tune across your band-of-interest in order to keep things peaked, but a single-frequency filter like this will give much greater rejection-of-nasties than simple high-pass filters.

[Image comes from "Secrets of RF Circuit Design" by Joseph J. Carr - ISBN 0-8306-3710-9 which is a really-good 'go-to' book full of ideas for us RF-types].

A couple of toroids, a 100+100pF ganged variable-capacitor, a sprinkle of support-capacitors - and you can consign anything more than a few hundred KHz off-tune to oblivion.

[Back in the 1970s this sort of thing was rather important to those of us who wanted to listen to radio-amateurs in the 7.0-7.1MHz band - because we were right adjacent to the 7.1-7.3MHz shortwave broadcast-band populated by Multi-Megawatt propaganda stations like Radio Moscow, Radio Tirana, Radio China and the like. Receiving a 100-Watt ham-station only 30KHz away from a Megawatt broadcaster was a bit of a challenge...]
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Old 5th Nov 2020, 6:56 pm   #6
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Default Re: Advice on the LOW PASS Filters

There is something odd going on here - and I doubt its going to be fixed by a filter in front of the receivers. For one thing these MW signals are being picked up on the tropical broadcast bands (4750 - 5060kHz I think?) and also LW (153 - 279kHz?) and apparently on a wide range of receivers, from vintage to modern.

Spurious signals generated in the receiver itself depends on non-linearities in the front-end. These are going to vary enormously from one set to another, particularly when comparing vintage with modern. Unlikely - in my view - unless in fact the OP is located very close to a MW transmitting site? In which case its possible what is happening here is simple overload or blocking of the receivers. Most receivers need hundreds of millivolts to suffer from this problem and that would require a close-by MW transmitter.

I am thinking its more likely that there is some non-linear device outside the receivers, which is generating a wide range of mixing products. It has to be a wide range - because its affecting both LW and 4 - 5MHz. My suspicion in the first instance falls on the "amplified antenna" that the OP mentions. That's going to be a wide-band amplifier without any tuning, presumably to match the antenna to a piece of coax? That could be generating such mixing products even if its not connected - with its output cross-coupling to the actual antenna in use.

My advice would be to

1. Switch off the amplified antenna - ideally remove it from circuit altogether, because it could still generate mixing products even when the power is removed. Then see whether the problem disappears.

2. If step 1 makes no difference, then take the IC-R75 receiver, and set it up to demonstrate the problem of a MW station being picked up on 4 - 5MHz. Then switch in the 20dB attenuator that I see is available on this set. What difference does that make to the unwanted MW signals?

3. If both steps 1 and 2 fail to make any difference, then give us some more information. Give us precise frequencies that you are picking up a specific MW station. Does the same station appear on both LW and 4 - 5MHz? If so what frequencies do you detect it? What MW station is it?


Richard
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Old 5th Nov 2020, 7:14 pm   #7
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Default Re: Advice on the LOW PASS Filters

And pulling up a list of MW stations (http://www.mediumwaveradio.com/uk.php) I see that the Westerglen site just south of Falkirk is in Stirlingshire (where the OP is located), and its putting out
  • 50kW for Radio 5 (909kHz)
  • 50kW for Radio 4 on 198kHz
  • 100kW for Radio Scotland on 810kHz
  • 125kW for TalkSport on 1089kHz
  • 125kW for Virgin AM on 1215kHz


That site is about 1km from the southern edge of Falkirk - so if the OP lives there, then I am not surprised there are problems with that lot above!
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Old 5th Nov 2020, 7:47 pm   #8
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Default Re: Advice on the LOW PASS Filters

Quote:
Originally Posted by G6Tanuki View Post
You could always build a multiplle-tuned-circuit tunable filter to add pre-receiver selectivity - this will be good at keeping-out _all_ unwanted signals whether they're above or below the frequency you happen to be wanting to receive.

It does mean you have another knob-to-twiddle as you tune across your band-of-interest in order to keep things peaked, but a single-frequency filter like this will give much greater rejection-of-nasties than simple high-pass filters.

[Image comes from "Secrets of RF Circuit Design" by Joseph J. Carr - ISBN 0-8306-3710-9 which is a really-good 'go-to' book full of ideas for us RF-types].

A couple of toroids, a 100+100pF ganged variable-capacitor, a sprinkle of support-capacitors - and you can consign anything more than a few hundred KHz off-tune to oblivion.

[Back in the 1970s this sort of thing was rather important to those of us who wanted to listen to radio-amateurs in the 7.0-7.1MHz band - because we were right adjacent to the 7.1-7.3MHz shortwave broadcast-band populated by Multi-Megawatt propaganda stations like Radio Moscow, Radio Tirana, Radio China and the like. Receiving a 100-Watt ham-station only 30KHz away from a Megawatt broadcaster was a bit of a challenge...]
Thanks for the advice and also the info of the book. I googled the book, and there was a FREE downloadable copy, so managed to get the book in PDF. It looks a great book for RF design and theories. Thanks.

I would see if I could build one by DIY from the book. Failing that, I will try to buy a commercially available filter.
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Old 5th Nov 2020, 7:57 pm   #9
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Default Re: Advice on the LOW PASS Filters

Quote:
Originally Posted by trh01uk View Post
There is something odd going on here - and I doubt its going to be fixed by a filter in front of the receivers. For one thing these MW signals are being picked up on the tropical broadcast bands (4750 - 5060kHz I think?) and also LW (153 - 279kHz?) and apparently on a wide range of receivers, from vintage to modern.

Spurious signals generated in the receiver itself depends on non-linearities in the front-end. These are going to vary enormously from one set to another, particularly when comparing vintage with modern. Unlikely - in my view - unless in fact the OP is located very close to a MW transmitting site? In which case its possible what is happening here is simple overload or blocking of the receivers. Most receivers need hundreds of millivolts to suffer from this problem and that would require a close-by MW transmitter.

I am thinking its more likely that there is some non-linear device outside the receivers, which is generating a wide range of mixing products. It has to be a wide range - because its affecting both LW and 4 - 5MHz. My suspicion in the first instance falls on the "amplified antenna" that the OP mentions. That's going to be a wide-band amplifier without any tuning, presumably to match the antenna to a piece of coax? That could be generating such mixing products even if its not connected - with its output cross-coupling to the actual antenna in use.

My advice would be to

1. Switch off the amplified antenna - ideally remove it from circuit altogether, because it could still generate mixing products even when the power is removed. Then see whether the problem disappears.

2. If step 1 makes no difference, then take the IC-R75 receiver, and set it up to demonstrate the problem of a MW station being picked up on 4 - 5MHz. Then switch in the 20dB attenuator that I see is available on this set. What difference does that make to the unwanted MW signals?

3. If both steps 1 and 2 fail to make any difference, then give us some more information. Give us precise frequencies that you are picking up a specific MW station. Does the same station appear on both LW and 4 - 5MHz? If so what frequencies do you detect it? What MW station is it?


Richard
You are spot on the analysis. Yes, I have been also trying the amplified ACTIVE antennas as well trying to copy the weak signals on the tropical bands at nights. And that is when 100% the MW breakthrough happens to all the receivers. It even happens other parts of the spectrum too. 9Mhz 15Mhz, but 4-6 Mhz is worst affected.

On the long wire, it is not that bad when on its own with ATU and preamp. But when the longwire is attached to the active amp, it is most severe with the MW breakthrough.

I have been trying out many tests with the antennas and receiving DX signals at nights, and my conclusion was that, with just normal antennas such as wires, dipoles, gp or loop whatever, it is not possible to DX most of the time unless the condition is very good.

But with the amplified devices and active antennas, I managed to copy some interesting DX time to time, more often than without them. Hence I am still trying to stick with them. But of course, there are problems with the cross modulations and overloading on the receivers with the setup.

I have been wondering if using good quality high pass filters could help in the situation.
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Old 5th Nov 2020, 8:01 pm   #10
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Default Re: Advice on the LOW PASS Filters

Quote:
Originally Posted by trh01uk View Post
And pulling up a list of MW stations (http://www.mediumwaveradio.com/uk.php) I see that the Westerglen site just south of Falkirk is in Stirlingshire (where the OP is located), and its putting out
  • 50kW for Radio 5 (909kHz)
  • 50kW for Radio 4 on 198kHz
  • 100kW for Radio Scotland on 810kHz
  • 125kW for TalkSport on 1089kHz
  • 125kW for Virgin AM on 1215kHz


That site is about 1km from the southern edge of Falkirk - so if the OP lives there, then I am not surprised there are problems with that lot above!
Yes, I am near the area, so all those stations are heard all over the LW MW and HF throughout when the active antennas and longwire is used.
I thought maybe the filters are only the option, or moving QTH. The former would be easier, quicker and cheaper option but would it work well effectively? I was not so sure.
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Old 5th Nov 2020, 8:05 pm   #11
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Default Re: Advice on the LOW PASS Filters

Many of the significant points in post 6 are a repetition of what I wrote in post 2.

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Old 5th Nov 2020, 9:36 pm   #12
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Default Re: Advice on the LOW PASS Filters

Initially I was more into resolving the cross modulation and overloading problems with the longwire, which happens time to time. The active antennas don't get used often. They are switched on for the last resort when the signals seems there but too weak or fading. But at that moment if the cross modulation or overloading appears, then it becomes no longer useful measure for the situation. If there were good filters capable of suppressing or clearing the over loading while keeping the wanted signal to be lifted out the noise, that would be great help.

But one thing I knew for sure was that none of the receivers were at fault.
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Old 5th Nov 2020, 9:59 pm   #13
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Default Re: Advice on the LOW PASS Filters

Quote:

Yes, I am near the area, so all those stations are heard all over the LW MW and HF throughout when the active antennas and longwire is used.
I thought maybe the filters are only the option, or moving QTH. The former would be easier, quicker and cheaper option but would it work well effectively? I was not so sure.
Filters will only work if the unwanted signals are being generated inside your receivers. For the reasons I have already given I think that unlikely.

If something outside your receiver is generating an unwanted signal on the 4 to 5MHz band then no amount if filtering will remove it. How could it? Filters only remove signals out of band, and the signals you want are in that same 4 to 5MHz band, along with the spurious signals.

The only cure is either to remove the non-linear element doing the mixing to produce spurious signals in the 4 to 5MHz band, or to move QTH. And a "non-linear" element might not be your antenna amplifier - it could be something as weird as a rusty fence or rusty bolt in a gutter. And those might not be near your house!

And sorry I have worse news horror you. I don't know what the harmonic levels are from those MW transmitters, but a quick check on ITU standards, suggest broadcasters can get away with 50mW maximum. Those MW transmitters I listed will have harmonics falling into your tropical broadcast band. Depending on exactly how far you are away from the broadcast transmitters, 50mW fed into what is probably a rather large broadcasting antenna could well be putting a substantial signal out over several miles. Again filtering at your receiver will do absolutely nothing to help in this situation.

Richard

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Old 6th Nov 2020, 8:58 am   #14
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Default Re: Advice on the LOW PASS Filters

We are planning to move to South of Glasgow area, but it will be next year.
Meanwhile, I will try to deal with the MW breakthrough to SW with filter or tuner device.

I find this article informative for the topic.

Here it says that when using the BCB filter, it will be clear whether the overloading is generated from the receiver or MW transmitter site.

https://pe2bz.philpem.me.uk/Comm/-%2...ce/rxtips.html

So does that mean, until you insert the BCB filter, the source of overloading is unknown?

"The BCB Rejection Filter is also an excellent way to determine if the signals heard are generated within the receiver or at a transmitter site. If the distortion signals disappear with the filter installed then they were generated within the receiver. If they are still present, then they are being generated at a transmitter site.

Another reason why a BCB filter is beneficial for any SW listening is because it eliminates the possibility of receiver desensitization. Strong BCB signals are allowed to pass through stages within the receiver which may create distortion. This reduces the receivers sensitivity even when the receiver is tuned to signals much higher in frequency. This is due to the fact that when an amplifier goes into distortion, the gain structure changes. An RF Amplifier with 10 dB gain may exhibit only 2 dB gain when subjected to a very strong signal. The noise floor of the receiver also increases in this condition.
"
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Old 6th Nov 2020, 3:28 pm   #15
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Default Re: Advice on the LOW PASS Filters

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So does that mean, until you insert the BCB filter, the source of overloading is unknown?
I think it's a near certainty that your set is suffering from intermodulation products caused by the strong signals from Westerglen.

Incidentally, "BCB filter" appears to be a marketing term used by the sponsor of that website, it's not a generic term like LPF, HPF, or BPF
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Old 6th Nov 2020, 3:35 pm   #16
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Default Re: Advice on the LOW PASS Filters

Quote:
Originally Posted by G6Tanuki View Post

[Though there are some examples of 'professional' radios - the receiver of the Pye C12 military transceiver, for example - where "MW Breakthrough" can be a real issue when working with long antennas]
This seems a good place to mention the R1155 "Athlone" filter.
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Old 6th Nov 2020, 4:41 pm   #17
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Default Re: Advice on the LOW PASS Filters

Two comments

The efficacy of a filter will depend upon WHERE the IMDs are being generated.

*IF* they are in your receiver(s) then filtering will help prevent front end IMD due to overloading

However the IMDs could be generated at the transmitter site and are being radiated in the band you want to listen to. Or as someone suggested somewhere else local to your receiver, the "rusty bolt" effect. These product will pass straight through the filter in your receiver

A quick look at the frequencies someone posted shows you are vulnerable to 5th and 7th order products

Secondly I would suggest you try, if you have space, a resonant dipole antenna for the band you are interested in rather than random or wide-band antennas

Good luck

Fred
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Old 6th Nov 2020, 5:55 pm   #18
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Default Re: Advice on the LOW PASS Filters

Quote:
Originally Posted by rambo1152 View Post
Quote:
Originally Posted by ORAWA01 View Post

So does that mean, until you insert the BCB filter, the source of overloading is unknown?
I think it's a near certainty that your set is suffering from intermodulation products caused by the strong signals from Westerglen.

Incidentally, "BCB filter" appears to be a marketing term used by the sponsor of that website, it's not a generic term like LPF, HPF, or BPF
Thanks for the correction. I thought BCB and HPF, LPF or BPF were the same thing. It depends on what freq. one was talking about, I used to think.
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Old 6th Nov 2020, 6:01 pm   #19
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Default Re: Advice on the LOW PASS Filters

Quote:
Originally Posted by dodgy-dxer View Post
Two comments

The efficacy of a filter will depend upon WHERE the IMDs are being generated.

*IF* they are in your receiver(s) then filtering will help prevent front end IMD due to overloading

However the IMDs could be generated at the transmitter site and are being radiated in the band you want to listen to. Or as someone suggested somewhere else local to your receiver, the "rusty bolt" effect. These product will pass straight through the filter in your receiver

A quick look at the frequencies someone posted shows you are vulnerable to 5th and 7th order products

Secondly I would suggest you try, if you have space, a resonant dipole antenna for the band you are interested in rather than random or wide-band antennas

Good luck

Fred
Thanks Fred. Great points.

For the fact that the filters are inserted between the antenna and receiver, I used to think that the problem was always in the antenna end. The antenna feeds with higher RF voltage than the receiver front end can cope, thus the cross modulation or over modulation occurs. The filters are to attenuate the voltage and feed with the refined RF to the receiver, I used to think.

The over loading in my receivers don't happen all the time, but sometimes usually in the evenings. Tonight, it happened again. I switched on the radio, and tuned to 4.7 Mhz, and there were these strong MW signals from 4.7 to all over to 5.2 Mhz. The antenna was a Chinese made MLA30 in the room. It was pointing to east and west. When it was moved around to point South and North, the MW breakthrough signals disappeared.
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Old 6th Nov 2020, 6:20 pm   #20
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Default Re: Advice on the LOW PASS Filters

Pragmatically, the receivers you have will _all_ have vastly-more-than-enough gain to handle the signals you are after, even when fed with a horribly-mediocre antenna.

The last thing you need to stick in front of them - specially when fed from an efficient antenna - is more gain. If anything you need selectivity/directivity - and attenuation!

I refer back to my obs on receiving a 100-Watt ham-station alongside Megawatt broadcasters on 40M: back then even with old-style valved comms-receivers [AR88, R1475, RA117] which had good strong-signal-handling abilities and 'tight' 3-signal-frequency-tuned-circuits in the front-end it was interesting to switch 3/6/9/12/18dB attenuation into the path between the antenna and the receiver.

If you add 10dB attenuation you lose 10dB of the wanted signal - but if it reduces unwanted intermodulation/splatter-products by more-than-10dB - you're still winning!

Have you considered a simple un-amplified ferrite-rod antenna tuned to the band-in-question? It'll give you some good directivity at least, and will probably deliver all the wanted-signal you need.
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