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Building a VHF receiver 108-136 Mhz

by Charybda
Tags: building, receiver
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Charybda
#1
Jan21-08, 06:13 PM
P: 3
Hi all,

I have a plan of building myself a VHF receiver that can pick up the air band (108-136 Mhz). I want to connect it than with an antenna that I plan to set it up on my rooftop.

Can anybody give me some working schemes and tips how to build such thing?

Thanks in advance.
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waht
#2
Jan21-08, 06:49 PM
P: 1,636
Your best bet is to build a super-regenerative receiver which is simple to construct, but is hard to tune.

http://www.jbgizmo.com/page23.html
berkeman
#3
Jan21-08, 07:39 PM
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The ARRL Handbook has receiver circuits that you can build for your application:

http://www.arrl.org/catalog/index.ph...ions+Reference

Also check out the QEX collection near the bottom of that page -- it might be of interest to you.

You might look up the local HAM radio club in your area, and stop by one of their meetings to ask about your project. They should be able to offer you some helpful advice.

Charybda
#4
Jan23-08, 09:55 AM
P: 3
Building a VHF receiver 108-136 Mhz

I appreaciate the comments. They helped me a lot. Now, a friend of mine adviced me to get a old VHS VCR, and he will adjust it and modify it so it will be able to work as a receiver of that band. Is this possible? He sounded like he knows what he is doing, but I am a bit sceptical.

Any advices?

Thanks in advance.
berkeman
#5
Jan23-08, 11:28 AM
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Quote Quote by Charybda View Post
I appreaciate the comments. They helped me a lot. Now, a friend of mine adviced me to get a old VHS VCR, and he will adjust it and modify it so it will be able to work as a receiver of that band. Is this possible? He sounded like he knows what he is doing, but I am a bit sceptical.
I'm skeptical as well, but maybe he knows some trick about the VCR pickup amp? I doubt it works in this frequency range, though.

I'd check out the local HAM resources, and look for an inexpensive kit to build. Also, ask about whether there are any local HAM flea markets -- you can pick up some amazing bargains there, as well as ask all sorts of people for their ideas.

BTW, be careful not to broadcast on those aircraft bands -- you generally need a special license to broadcast (pilots get those special radio licenses).
Charybda
#6
Jan23-08, 01:53 PM
P: 3
No, my idea is only to pick up the freq and not broadcast anything on those freq. I talked with another guy who said that it might be possible, so tomorrow I will search for a used VCR that we can work on. My local area doesn't have HAM flea markets at all, so it is very difficult to get my hands on things like this.

Anyhow, about the VCR, he said that with turning off the video signal, and leaving the sound (the band is VHF, Channel 3 or Channel 4 which converts to around 112 Mhz and more) so this way this band would be in the same range. Well, if anything, I will let you know if this works.

Thank you.
waht
#7
Jan23-08, 10:12 PM
P: 1,636
If the VCR has a cable tuner then you can modify it for a receiver from 50 to 550 MHz. For that you would have to construct a voltage variable power supply from 0 to 30 volts to tune the tuner.
NoTime
#8
Jan23-08, 10:43 PM
Sci Advisor
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P: 1,571
The FM radio band and the Aircraft band fall between Standard TV channels 6 and 7.
I know you can modify a FM reciever and the VCR idea seems viable.
The Aircraft channel spacing is small 25Khz, so if you are in an area with a lot of air traffic the IF circuit bandwidth may be too wide.
sas3
#9
Jan25-08, 02:20 AM
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Here is a design that covers 60-150MHz that you may want to look at.

http://www.users.bigpond.com/cool386/6tr/srrx.html
radio_newb
#10
Mar24-08, 01:58 AM
P: 11
I'm trying to receive the same frequency's, I have an old Sony Air 7 Tuner/Receiver, with a 1' or so Helical antenna(RX only I assume), problem I'm having is hearing both sides of the conversation, I've moved the antenna outside onto my deck, with some BNC to RJ45 connectors and a bunch of cat 5 cable, so its at about 10' off the ground, in a fairly open area. My reception is much better, very clear, but still no ground level traffic on the air band, and minimal range on the P.S. Band, about 5 to 10km at most.

I'm going to mount it on the roof, and see what, if any gain I get, but that is at max 10 more feet, and I know the terrain in the area is probably not optimal, so that is a factor I am looking to overcome as well.

Does anyone have a design for a simple antenna that I could make around 6 to 8 feet high or less, that would extend my listening range??(something smaller, don't want to piss off the neighbors) Maybe something directional for the Air Band, I know the exact direction from my area, its roughly 30km away.

Or is this something that is just a matter of height and distance, I'll string the antenna to a tree if it makes a difference :)
berkeman
#11
Mar24-08, 11:27 AM
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Can you try a simple TV antenna? Do you have one on your rooftop already? It might have enough directional gain and is close to the frequencies you mention. You might need to rotate it vertical, though -- what polarization is used for aircraft communications?

If that doesn't work well enough, you can make yourself a multi-element Yagi antenna to boost your gain (but you will need to narrow your bandwidth requirement). You can pick up good home-brew antenna books at your local HAM radio store. The HAM Radio Outlet has stores in many areas.
radio_newb
#12
Mar24-08, 12:11 PM
P: 11
when you say a TV antenna, you mean a single mast, say like off the back of a late 80's model tv, or stereo, straight, extend-able, kinda like a professor's metal pointer, or are you talking something like rabbit ears or something else?

as for polarization, I'm not really sure what that is :) If you explain it, maybe I can answer.
berkeman
#13
Mar24-08, 12:15 PM
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Quote Quote by radio_newb View Post
when you say a TV antenna, you mean a single mast, say like off the back of a late 80's model tv, or stereo, straight, extend-able, kinda like a professor's metal pointer, or are you talking something like rabbit ears or something else?

as for polarization, I'm not really sure what that is :) If you explain it, maybe I can answer.
Yeah, like this one on a chimney:

http://www.granbytv.com/3671__Antenna.jpg

That is the classic multi-element TV antenna, covering the VHF and UHF bands (the longer elements are for the VHF band). The spacings are meant to give the antenna good gain in the front-to-back direction. Since the elements of the antenna are horizontal in the picture, that antenna is meant to receive horizontally-polarized signals (the electric field of the plane electromagnetic wave is horizontally polarized).

You can tell the polarization of the transmitting antennas that you are trying to receive by the orientation of the antenna. If it is like a vertical stick on the plane, then it is transmitting a vertically polarized main signal. Reflected signals can have pretty much any polarization, depending on what they reflect off of.
zeitghost
#14
Mar24-08, 01:43 PM
P: 157
Also remember that VHF signals are pretty much line of sight... it may be that you are just too far away from the control tower to receive the signals at all.

The height of your receiving antenna can also have a profound effect on the amount of signal you receive, higher is usually better... (but not always so).
radio_newb
#15
Mar24-08, 02:52 PM
P: 11
well, I don't have one of those great huge things, I thought you meant the small antenna's that would come out of the back of the TV like this :

http://www.thesourcecc.com/images/On...7/2701402l.jpg

Still waiting on an 8' ladder to get up on the roof, to mount my original helical antenna that came with the radio, I'm hoping that helps. Link to the radio :

http://www.rigpix.com/sony/sony_air7_manual.pdf

Is it helpful/possible to have 2 or 3 small antennas all fed in thru Coax splitters, to one tuner?

Also, I ran some new cables today, all 75ohm Coax, the piece I have is about 50' too long, but now with the antenna placed in a temporary location, well below the horizon, and the excess cable just laying on the lawn, I seem to be getting much better reception, and I think I'm getting some of the tower chatter, does that make any sense???

and thanks for the response's, and ideas, if you have a computer/network question, I'm your guy :)
berkeman
#16
Mar24-08, 03:48 PM
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Quote Quote by radio_newb View Post
well, I don't have one of those great huge things, I thought you meant the small antenna's that would come out of the back of the TV like this :

http://www.thesourcecc.com/images/On...7/2701402l.jpg

Still waiting on an 8' ladder to get up on the roof, to mount my original helical antenna that came with the radio, I'm hoping that helps. Link to the radio :

http://www.rigpix.com/sony/sony_air7_manual.pdf

Is it helpful/possible to have 2 or 3 small antennas all fed in thru Coax splitters, to one tuner?

Also, I ran some new cables today, all 75ohm Coax, the piece I have is about 50' too long, but now with the antenna placed in a temporary location, well below the horizon, and the excess cable just laying on the lawn, I seem to be getting much better reception, and I think I'm getting some of the tower chatter, does that make any sense???

and thanks for the response's, and ideas, if you have a computer/network question, I'm your guy :)
Are you using matching networks/baluns between your antenna and the coax? The manual is not very clear whether the antenna input jack on your radio is 50 or 75 Ohms -- do you know? You should try to use matching networks wherever there is a transition (like form 75 Ohm cable into a 50 Ohm input jack).

The antenna shown in the manual for Aircraft is vertically polarized (the one on the left), which makes sense. What frequency are you picking up the tower on? 150-ish MHz or so?

The best and simplest directional antenna that you can make is a 3-element Yagi, which is basically a simplified version of the multi-element antenna shown in the chimney picture that I linked to. It looks like this:

http://www.mikesfalconry.com/images/...0open%20lg.jpg

The actual antenna is the middle pair (like two sides of the simple TV antenna you showed, with the telescoping length of the two sides set based on the frequency you want to tune to. The spacings to the front "director" pair and the back "reflector" pair, and the lengths of the director and reflector elements can be calculated using this calculator:

http://www.csgnetwork.com/antennae3ycalc.html

You would build the 3-element Yagi to the specs from the calculator, and then mount it up high in the vertical orientation like is shown in the hand-held version picture. Aim it at the airport, and you should get a pretty good signal.
radio_newb
#17
Mar24-08, 08:30 PM
P: 11
Ok, lets see, tower is 118.4mhz, plus Im trying to get ATC for the area on 119.2mhz, I'm pretty sure the jack on the radio is 75, but not certain, when I switched over to all 75ohm coax i did see a decent improvement in quality of sound, and it seem to pick up more traffic.

Also, while waiting earlier, I went ahead and built this antenna(2nd one on page, link below), without the fancy coil, and actually I'm picking up alot more then I was with the factory antenna, I can't wait to try it on my roof. Its very simple, but it seems to be working so far, have a look.

http://users.belgacom.net/hamradio/s...rubberduck.htm

I basically added a cheap old stereo antenna like in the pick in my last post, from an old boom box I had kicking around, did a bit of soldering, and bango, now I have tower talk, and its a CHICK LOL :) Would the coil he uses help me at all ??

I've been playing with the angle and direction, and so far it seems to be working the best when the antenna is straight up and down, as high as I can get it, for now... I asked this before, does the length of the coax feed line from the hand-set to the antenna make any difference??
radio_newb
#18
Mar24-08, 08:32 PM
P: 11
Would it make a difference putting the antenna on the chimney like in that picture?? Or would the furnace create interference ?? I live in a cold climate, furnace runs a lot :)


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