FM Leakage on Airband

[veedub565]

Hi,

I built my own airband receiver, for fun, based around an old Ramsey kit. It works surprisingly well sat on the windowsill at work in Telford, I can get the Clee Hill repeater quite well on 133.6Mhz.

When I take it home to Walsall though it doesn't work so well, and all I can get is lots of distorted FM. I can't really get the Clee Hiill repeater and only sporadically get the aircraft themselves.

I suspect that at home I'm just plain out of range of Clee Hill. I'm also within direct line of sight of the 250kw Sutton Coldfield transmitter, maybe 3 or 4 miles and it's overloading the front end ?

Sound like a reasonable assumption? and is there anything I can do about it?   I'm using a homebrew dipole tuned to the airband freq.

[vk3yedotcom]

There's a couple of things we need to know first.

1. What type of receiver is the Ramsey? A superhet, superregenerative or straight TRF?

2. Does the offending signal appear all over the dial or at one spot on it?

You mentioned there were two main problems - (a) the breakthrough from the strong local FM station and (b) the poor reception.

a. FM breakthrough

If the offending signal appears on one part of the dial AND it's a superhet receiver then the problem is likely to due to poor image response.   That is there is an unselective front end which is causing FM signals to break through.  Bear in mind that an FM broadcast station may be 50 kW while aircraft band signals may be 1/1000 the power or so (as that's all they need). 

Otherwise, if the offending signal appears all over the dial then it's likely to be direct breakthrough or overload.  In extreme cases detection can happen in the audio amplifier but it's likely to be in the RF portion.  You need some way of reducing its strength while keeping the strength of the desired signals. 

Since the problem is coming from only one station a notch filter is probably your best bet.  This is a series tuned circuit connected across the antenna connections.  The tuned circuit is set to the frequency of the offending FM station and adjusted for a null.  Hopefully as it's 20 or 30 MHz away reception of aircraft band signals should be unaffected.  As a starting point I would suggest 4 or 5 turns wrapped around a pencil with a 30 or 40 pF trimmer capacitor in series to resonate on the FM b/c band.

Other treatments include a narrow bandwidth antenna (eg a magnetic loop say 10-15cm diameter).  However this will need adjusting (along with the receiver tuning) for greatly different frequencies in the aircraft band and may not be as sensitive so is not a good option.  Or consider a multi-section high pass filter at 108 MHz.  Though you will need lots of sections to offer reasonable attenuation since the FM band is just below that.  Or if the breakthrough is getting into the audio amplifier then a shielded metal box for the receiver may help.

b. Poor reception.  Ramsey kits were often pretty rudimentary so you might just have to accept this. But a small beam antenna may help - say 2 or 3 elements.  A potential benefit for this is that a 2 el beam has a good rejection off the sides so you may be able to turn it so that the offending signal is minimised.  But it will not necessarily then be in the best position to hear the aircraft.

[veedub565]

Thanks for all the info

I think it's a straight TRF, schematic here http://n5dux.com/ham/kits/Airband-Receiver/schematic.jpg

The offending signal appears to be multiple FM stations rather one single signal, and they appear at various points around the dial. It's a bit like when you tune an FM radio.

[CJay]

Nope, that's a superheterodyne, there's a local oscillator which is tuned by the BB910 varicap and mixed with the incoming 'wanted' signal in the NE602.

As vk3yedotcom says, it'll have poor image rejection and the front end will be being swamped by the strong local signals which will desensitise it to wanted signals.

I suspect it'll be using a brown, ceramic 10.7MHz filter for the first IF, I think it'll be low side injection for the mixer as well so the LO will be running 10.7MHz below the wanted signal.

That means it''ll have an image response which is in the VHF broadcast band so that'll explain why the FM stations appear across the dial (if you've a frequency counter you might be able to work out how to tune FM stations by measuing the LO and subtracting 10.7.

You need a high pass filter on the front end which will go some way to reducing the sensitivity to VHF broadcast (but if you've got 250KW three miles away it can only do so much)

I'd experiment with the LO to try and get it above the wanted band (which might be difficult as the coils are etched into the PCB I think?) and then use a narrow 21.4MHz crystal filter for the first IF (I've got some second hand ones sat here if you want to try one) though it's never going to be a great receiver it could be a lot of fun to tinker with.

[eb4fbz]

Looks like overload and image/spurious response. 10.7MHz IF looks too low IMHO, for airband is preferably to use 21.4 or 45MHz.

[Nemo1956]

I have two of these kits. One with proper coils in the frontend filter section and oscillator and the other one has etched coils on the PCB. This one is rubbish and I have never got it to tune over the whole air band but the first one with proper coils will work fine but as you have stated it get over loaded with local radio stations.
I have also noted that the whole thing is very unstable and will self oscillate so I put it into a nice metal box and the sorted that out.
It need a better filter at the front end and a better IF filter as well to make it work as it should. But being fair to it it was only intended to be used what at an airport and not from home.
I will make some changes to it and post them on here so all can do.

Nemo.

[babysitter]

I suggest posting a image of the setup at least with RF questions. Two very simple things you can try:
a) get a attenuator in front of this thing (either broadcast type or resistors in Pi configuration, does it help?
b) make a pi filter out of quarter-wave coax stubs. The difference is small but possibly it works.