Possible to do a temporal homebrew filter? And a few other questions

[Luis_C]

Hey guys, I monitor aircraft transmissions, I use a Pro-95, and a center loaded telescoping antenna for now. But I have noticed, that the Pro-95, is really sensitive to interference of all kinds, most of the frequencies I listen to, at about 48% schelch, I listen to all kinds of strong interferences, almost everyplace that I monitor, and really crappy signal, sometimes there's luck and works ok, it would always work ok, so I believe a AOR ABF125 filter would really help, problem is, I can't order here to Mexico, it will become tad expensive, and the customs here when they see weird stuff, they don't let it pass, and I'm not sure if Universal Radio would ship it for me. But I have student friends, currently not there because of vacations, that could receive it for me, but that would be in a few months.

So I was wondering if I could do a bandpass filter but homemade, I don't really care if it gets a big box, because I almost do all my monitoring being on base, (simply I mean that I get all my stuff in some place, and I sit there and enjoy, so carrying something extra doesn't matter.) and if it won't work as good as the AOR, but if it will help that's great.

I saw this: http://ourworld.compuserve.com/homepages/Bill_Bowden/opamp.htm#bandpass.gif Wonder if it would work, I'd do it but I'm confused with the formulas, I don't know what Q and Gain should I use, if someone can give me some help I'd really appreciate it or another ideas would be great.

What I also noticed is, that going near my airport, or when there's good propagation is that I can pickup the center that controls the sector here, that isn't controlled by airports clearly, actually the center is at Mazatlan, 400 miles from here, giving instructions to airplanes that are really far of my city, I sometimes can't pick them up, but I have a clue of what's going on with the traffic, here the pilots sometimes don't repeat instructions, or they say it really fast, or they kinda abbreviate it, but the controllers know that they are right, so pilots are harder to understand. A guy also has noticed that in major airports his reception is great, so he asked to pilots and stuff, and there seems to be some kind of repeater or something, that helps. And personally here, listening to the Center, and Tower, complete fun.

So I was thinking that I could buy a TV VHF Yagi Antenna, and modify it for Airband, changing the elements length, separation and everything, get it as tall as I can, (security first) for pickup the airport. Luckily, there's not hills nor tall buildings that could affect, and it's only about 6 miles.

What do you guys think?

Thanks

Edit: I already tried a home made ground plane antenna, I didn't had really good results, I was thinking that I should better do a J-Pole, because it is really popular for airband and it provides 3dB of gain, when the ground plane doesn't provide any, but I have to get tools for be able to solder and stuff, that shouldn't be much of a problem, but probably the Yagi solution would be the easiest one.

Edit2: It would be easy for me to buy a CB antenna, do you think it would work crappy, or it has a chance?

Thanks

 

[ka3jjz]

Luis - if I'm understanding this correctly, what you are describing is a typical limitation of using handhelds with larger antennas - it's what we call overloading. It's very likely that the various antennas you are trying are simply too much for the radio to handle - resulting in a lot of garbage that you really don't want to hear.

Putting a 'CB' or Yagi antenna on the radio would only make the problem worse, not better. The fact is that handhelds simply cannot handle the amount of energy that larger antennas would deliver. They're not designed for it. A ground plane - or perhaps, at best, the JPole you mentioned earlier - is probably the best you could expect under these conditions, though I have my doubts that a JPole would work well across the entire VHF civil air band (it might work quite well on just a part of it, though - there is at least one website on our Wiki that sells such an antenna, so it is possible). Even the JPole might well be too much.

A bandpass filter like the AOR is precisely what you would need. It might even be possible to construct one, given the availability of parts and a good shielded metal box in which to put the filter. Perhaps a better bet is a better scanner - one that is more resistant to overloading. Again, being in Mexico, something like that may be problematic at best, with customs duties, fees and other things being added onto something that might already be kinda expensive, particularly when you do the conversion from US Dollars.

Sorry to be a bit pessimistic on this, but it's the only thing I can think of at the moment.

 

[pro92b]

The bandpass filter article you referenced is for audio, not RF. Visit http://www.parelectronics.com/index.html and send them a note asking for a price on a bandpass filter for the air band.

A CB antenna is not good for your purpose since it is made for frequencies far from the air band. At http://www.dpdproductions.com you will find antennas specifically for the air band. I have not used their products myself and am just passing along the link as information.

 

[ka3jjz]

The bandpass filter article you referenced is for audio, not RF. Visit http://www.parelectronics.com/index.html and send them a note asking for a price on a bandpass filter for the air band.

If you are referring to the AOR filter as shown here...

http://www.universal-radio.com/catalog/filters/0880.html

This is most definitely an RF filter, and has absolutely nothing to do with passing audio frequencies. In this case, this filter will let civil air freqs through, but attenuate everything else - hence the name 'bandpass'. The description even warns about this attenuation.

A CB antenna is not good for your purpose since it is made for frequencies far from the air band. At http://www.dpdproductions.com you will find antennas specifically for the air band. I have not used their products myself and am just passing along the link as information.

While I agree DPD makes very good antennas, they will simply aggrevate the overloading he is experiencing. Remember he is using a PRO-95 - If he was using, say, a BC780, I'd say go for it. But certainly not in this case.

This is one case - among many, when working in urban type environments, when less antenna really is more.

73s Mike

 

[Luis_C]

If you are referring to the AOR filter as shown here...

http://www.universal-radio.com/catalog/filters/0880.html

This is most definitely an RF filter, and has absolutely nothing to do with passing audio frequencies. In this case, this filter will let civil air freqs through, but attenuate everything else - hence the name 'bandpass'. The description even warns about this attenuation.

Yes, that's the one. And I agree with you guys, there's limitations, and there's not much you can do, yes I do know that there's some attenuation, but probably it will work better.

What I also found for a home made filter is this: http://www.raltron.com/cust/tools/band_pass_filters.asp And now that looks like for RF. And yes, it says that it is for educational purposes only, but there's a reference.

So well, I calculated this:

Impedance = 50 Ohms (Because it is for the radio scanner)
Frequency = 122 Mhz (Aprox the center for Airband)
Quality = 10 (How much does it attenuate, 10 = 24 dB aprox)

C1 and C3 = 260.9 pF
C2 = 1.3 pF
L1 = L3 0.007 µF
L2 = 1.3 µF

Now with a quality of 5 = 18dB

C1 = C3 130.5 pF
C2 = 2.6 pF
L1 = L3 0.013 µH
L2 = 0.65 µH

Quality = 1 = 6 dB

C1 = C3 26.1 pF
C2 = Infinity pF
L1 = Infinity µH
L2 = 0.13 µH

Do you think it has a chance?

If so, I'll see if I can get the components for one that in theory would ATT better. That's the first one, and I have a friend whom his dad repairs electric stuff, so he probably might have similar capacitators around, so I can experiment with different ones and see what comes to work better. But still, I will do my best for getting the AOR, probably is the best I can do.

Ok, and now dreaming what scanner would you guys get for airband listening? A communications receiver probably, right?

Hehe, even if I had the money, parents would believe that I'm crazy.

Well, thank you all.

 

[pro92b]

If you are referring to the AOR filter as shown here...

http://www.universal-radio.com/catalog/filters/0880.html

This is most definitely an RF filter, and has absolutely nothing to do with passing audio frequencies. In this case, this filter will let civil air freqs through, but attenuate everything else - hence the name 'bandpass'. The description even warns about this attenuation.

No, I referred to the article on : http://ourworld.compuserve.com/homep...m#bandpass.gif which is an audio filter. I looked at the AOR filter and still recommend contacting PAR to get something better. A tuning range of 118-136 would allow for some FM broadcast attenuation which may be critical in reducing overload.

While I agree DPD makes very good antennas, they will simply aggrevate the overloading he is experiencing. Remember he is using a PRO-95 - If he was using, say, a BC780, I'd say go for it. But certainly not in this case.

This is one case - among many, when working in urban type environments, when less antenna really is more.

73s Mike

I don't agree with your contention that the antenna would cause more overload. It is likely that signals outside the air band are responsible for overload. A tuned antenna will be less efficient outside the air band and should help not hurt. It also seems that the signals from the airport are weak at his location. The right filter plus an antenna tuned for the air band will have the best chance of success. Broadband antennas like the discone are intermod magnets and should not be considered for this application.

 

[ka3jjz]

A communications receiver?? Nah - since you are using a PRO-95, this leads me to believe that you're not interested in - or don't have - digital trunking as we do here in the States.

To that end, a BC780 or even a BC895 will serve you quite well. I've seen several on various boards for under USD200. Heck, I've even seen 895s for much less than that - around USD125 or so.

I would start by doing some research on the web - the well-known Strong Signals website has a models FAQ, along with some reviews on these older models. EHam has numerous reviews as well. This will allow you to make a more informed choice.

One caution I would make - even a good base rx like a BC780 may experience overloading using large antennas in urban settings. You may well still need a good bandpass filter - assuming the civil air band is all you're interested in - or other filters (such as those sold by Par Electronics, as PRO92b has mentioned). While it's generally true that they resist overloading better than their handheld cousines, it can - and does - still happen in such RF rich environments.

73s Mike

[edit] The Capitol Hill Monitors - A DC based group - has a nice selection of links that can help you with your research at:

http://henney.com/chm/links/scanners.htm#REVIEWS

and we have the URL for PAR electronics, and a few other filter manufacturers on our Scanner Antennas wiki;

http://wiki.radioreference.com/index.php/Scanner_Antennas

 

[ka3jjz]

I don't agree with your contention that the antenna would cause more overload. It is likely that signals outside the air band are responsible for overload. A tuned antenna will be less efficient outside the air band and should help not hurt. It also seems that the signals from the airport are weak at his location. The right filter plus an antenna tuned for the air band will have the best chance of success. Broadband antennas like the discone are intermod magnets and should not be considered for this application.

Depending on the antenna design, I might agree. Given that he is experiencing serioius overload problems even with a center loaded whip, a Yagi or something similar would certainly improve the air band reception; however, Yagis tend to be better at ground based units, and probably wouldn't be all that effective at hearing aircraft at some altitude. Yes, you'd hear some, but probably not as much as with something like a ground plane.

And while the frequency response outside the designed band may well supply enough attenuation, I suspect it would not be enough. After all, many antennas - depending on the design, of course - will also work at multiples of their design frequency, with some loss of efficiency and/or directivity. I would still tend to suspect a larger antenna would increase his overload problems - perhaps not in the air band, but other places in the spectrum would almost surely suffer.

However, I do agree that a tuned antenna, with a good bandpass filter, is the best formula for success. We just have to keep in mind that a handheld isn't going to be very happy with any big antennas - even those designed for a single band - without experiencing some chances with overloading elsewhere, without additional filtering.

73s Mike

 

[Luis_C]

A communications receiver?? Nah - since you are using a PRO-95, this leads me to believe that you're not interested in - or don't have - digital trunking as we do here in the States.

To that end, a BC780 or even a BC895 will serve you quite well. I've seen several on various boards for under USD200. Heck, I've even seen 895s for much less than that - around USD125 or so.

I would start by doing some research on the web - the well-known Strong Signals website has a models FAQ, along with some reviews on these older models. EHam has numerous reviews as well. This will allow you to make a more informed choice.

One caution I would make - even a good base rx like a BC780 may experience overloading using large antennas in urban settings. You may well still need a good bandpass filter - assuming the civil air band is all you're interested in - or other filters (such as those sold by Par Electronics, as PRO92b has mentioned). While it's generally true that they resist overloading better than their handheld cousines, it can - and does - still happen in such RF rich environments.

73s Mike

[edit] The Capitol Hill Monitors - A DC based group - has a nice selection of links that can help you with your research at:

http://henney.com/chm/links/scanners.htm#REVIEWS

and we have the URL for PAR electronics, and a few other filter manufacturers on our Scanner Antennas wiki;

http://wiki.radioreference.com/index.php/Scanner_Antennas

Well, just analog trunking on some big cities, but I believe that in cities with high rate crime there is. But the higher rank agencies, have that Digital GSM Servive running around 400 Mhz, I don't remember how's called.

I'm not simply really a fan of listening much of other things besides Airband.

Depending on the antenna design, I might agree. Given that he is experiencing serioius overload problems even with a center loaded whip, a Yagi or something similar would certainly improve the air band reception; however, Yagis tend to be better at ground based units, and probably wouldn't be all that effective at hearing aircraft at some altitude. Yes, you'd hear some, but probably not as much as with something like a ground plane.

And while the frequency response outside the designed band may well supply enough attenuation, I suspect it would not be enough. After all, many antennas - depending on the design, of course - will also work at multiples of their design frequency, with some loss of efficiency and/or directivity. I would still tend to suspect a larger antenna would increase his overload problems - perhaps not in the air band, but other places in the spectrum would almost surely suffer.

However, I do agree that a tuned antenna, with a good bandpass filter, is the best formula for success. We just have to keep in mind that a handheld isn't going to be very happy with any big antennas - even those designed for a single band - without experiencing some chances with overloading elsewhere, without additional filtering.

73s Mike

Well, not overload problems with the center whip here at my location, (simply better than the ground plane, and the stock antenna, and the center whip, all the top sides oppened, at the bottom nothing but still the signal of the airport is weak), but there's some really high rf places, where it simply doesn't work.

Yes, I agree. I'll try to get a filter as fast as possible, but I understand that there's limits in the handhelds.

Thanks again

 

[Luis_C]

A communications receiver?? Nah - since you are using a PRO-95, this leads me to believe that you're not interested in - or don't have - digital trunking as we do here in the States.

To that end, a BC780 or even a BC895 will serve you quite well. I've seen several on various boards for under USD200. Heck, I've even seen 895s for much less than that - around USD125 or so.

I would start by doing some research on the web - the well-known Strong Signals website has a models FAQ, along with some reviews on these older models. EHam has numerous reviews as well. This will allow you to make a more informed choice.

One caution I would make - even a good base rx like a BC780 may experience overloading using large antennas in urban settings. You may well still need a good bandpass filter - assuming the civil air band is all you're interested in - or other filters (such as those sold by Par Electronics, as PRO92b has mentioned). While it's generally true that they resist overloading better than their handheld cousines, it can - and does - still happen in such RF rich environments.

73s Mike

[edit] The Capitol Hill Monitors - A DC based group - has a nice selection of links that can help you with your research at:

http://henney.com/chm/links/scanners.htm#REVIEWS

and we have the URL for PAR electronics, and a few other filter manufacturers on our Scanner Antennas wiki;

http://wiki.radioreference.com/index.php/Scanner_Antennas

Well, just analog trunking on some big cities, but I believe that in cities with high rate crime there is. But the higher rank agencies, have that Digital GSM Servive running around 400 Mhz, I don't remember how's called.

I'm not simply really a fan of listening much of other things besides Airband.

Depending on the antenna design, I might agree. Given that he is experiencing serioius overload problems even with a center loaded whip, a Yagi or something similar would certainly improve the air band reception; however, Yagis tend to be better at ground based units, and probably wouldn't be all that effective at hearing aircraft at some altitude. Yes, you'd hear some, but probably not as much as with something like a ground plane.

And while the frequency response outside the designed band may well supply enough attenuation, I suspect it would not be enough. After all, many antennas - depending on the design, of course - will also work at multiples of their design frequency, with some loss of efficiency and/or directivity. I would still tend to suspect a larger antenna would increase his overload problems - perhaps not in the air band, but other places in the spectrum would almost surely suffer.

However, I do agree that a tuned antenna, with a good bandpass filter, is the best formula for success. We just have to keep in mind that a handheld isn't going to be very happy with any big antennas - even those designed for a single band - without experiencing some chances with overloading elsewhere, without additional filtering.

73s Mike

Well, not overload problems with the center whip here at my location, (simply better than the ground plane, and the stock antenna, and the center whip, all the top sides oppened, at the bottom nothing but still the signal of the airport is weak), but there's some really high rf places, where it simply doesn't work, not in my house though.

Yes, I agree. I'll try to get a filter as fast as possible, but I understand that there's limits in the handhelds.

Thanks again

 

[Luis_C]

Hey guys, do you think that if I can get an AOR 8000 in eBay would work well for my requeriments? (Nice airband reception, support external antenna well) I'm excluding the Uniden ones a bit, because I would love to enter to the world of Low Band DXing, and I would still like something portable. I know that I'm asking too much, but if it would in theory work better than the Pro-95, by something considerable, in Airband, and VHF, and also a bit of Dxing, I'm looking for that.

Thanks

Edit: Or wait, maybe I could get an used Uniden, and then some old vintage shortwave receiver, with each one with a different antenna, would that be better? I think so

Recommendations for shorwave receiver in budget? Guess I'm using $200 for the Uniden, and then get another $150-200 for the shortwave, not counting antennas.