Breakthrough from Firebrigade on MOST 4m channels, pls help

[G7IVJ]

Gentlemen,

I am living in Surrey south london about 525 foot ASL - using a Simoco FM1200 with the PA4DEN conversion. Installed with 13metres of Westflex 103 feeding the Sirio CX-468 3/4wave end fed vertical up about 10metres on the telescopic mast.

I am experiencing breakthrough which is anything from S0 to S9 on at least every other freq all the time on 4m from 70.00 right upto 70.500 which obliterates most of FM my QSO's on 4m.

We have plugged in another radio (Motorola) to see if it reduces the problem, but it does not.

The breakthrough is not on each channel all the time, it is completely random and can last a second or 20minutes.

Listening around we have located it to London and Surrey Fire Brigade AM breakthrough.

Any ideas how I can help eliminate / reduce this problem?

Thanks for your help

Best Regards, Simon

[EI9GQ]

Are Surrey fire brigade still using AM?

You need to find out what frequency they are using and if the base station or repeater is very close to where you live. This list might help:
http://scanuk.9.forumer.com/a/uk-scanni ... ost17.html

It may be possible to use a BPF, notch filter or a combination of the two to suppress the interference.This is not too difficult if the interfering signal is at a frequency which is well away from 70MHz. If the interfering signal is close in frequency, it won't be easy to filter it out. If the problem is caused by overloading of your RX front-end, a simple attenuator will probably help. Another option is to use a directional aerial, although Murphy's Law dictates that the signal you are trying to receive will always be from the same direction as the interfering signal. Using a horizontal aerial would probably help a lot, but since most FM stations are using verticals, this might not work well in practice.

Good Luck.

Ed. EI9GQ.

[PA5DD]

Sounds to me like in-band intermodulation ie. two carriers in the 70 MHz band mixing to produce the channel you are listening to. I experienced something similar on 70cm when I lived next to Copenhagen airport. They had a number of carriers in the air between 430-432 MHz.

You will need a very narrow bandpass filter to filter it out. The best thing to try is the trick with the attenuator.

First try to figure out the frequencies used by the interferers. If they are out-of-band, that will help a lot.

73 Uffe PA5DD

[G3PTU]

I suspect that these 'other users' are in fact just above the 70Mhz band - so they could be very close to the frequency you want to use. Use a scanner to see where they are.
I assume you you are very close to one of these operaters you may know this.
First check all the connections are tight on your antenna, plus are soldered etc.
If its cross mod in the front end you have, this may present a problem
Try the 10db atenuater metioned, if this removes it well and good, but remember your rx in 10Bd deafer.
Lets hope you have say a situation wherbey the interfernece is at the top end on the next (say) 500Khz. You can either try bandlimiting your reception to say stop at 70.475+ or a notch at the other fellows frequeny
Whichever way you need a filter, for band pass this will probalby have to be at least 2 element if not more - have look at rallies there are some ex comercial bandpass filters about. These are usually in a metal box with (say) N conecters.
If its more than 3 elements setting it up can be a nighmare.
Notches are quite big at 70Mhz, [note there are no repeaters] you could try making one with water pipe or the like, but the problem is the Q needed is big and the merit index is poor i.e 70/0.5.
The fact that 2 rigs are the same seems to suggest that interfence is very local to you and not your rig.
Which brings me to another type of problem. That of problems on the other fellows mast. Sometimes poor joints bolts etc on a mast cause in efect non liniear problems. Like wise, if Rf from one tx gets back in to another tx down the coax reversely at it where.
If possible look the end of your feeder with a spectrum anyliser and see if there is products present within the band. This should also give some idea of quite what and the ratio of what you complain about. [The 10db pad thing should give a clue if the problem is external to yourself].
If its present incoming to you; in band there is nothing you can do, you need to speak to the other party. But remember they are higher priority than you are. Never the less you might stike it lucky especially if you are some distance from their site - argument 'unreasonable interference'. To find this type of problem takes time, one I witnessed took 3 days of work examining every bolt with a dipole made out wire connected to a spectrum anyiliser at ground level. Eventally one was found radiating the spririi. You will be up against - acountants - lethagy - ignorance and 'go away'
David G3PTU

[G8YMW + M0LUY]

I think Uffe is right, I've known this phenomenon on 2 metres near Lincoln Castle. On the Cathedral is GB3LM and the operator had 20dB over noises throughout 2 metres whenever GB3LM fired up. The radio was a Yaesu FT221 with Mutek board. A bandpass filter reduced the sprogs. When a BT Investigation Officer called round he told us of a BT pager recently installed in the Cathedral (This was the mid 80's) Turned out it was mixing in the receiver. Mutek took the filter and modded it to give greater out-of-band suppression which sorted it.
So Andy try a bandpass filter. An Attenuator can be used to prove it because the interference will be reduced by a disproportionally large amount if it is intermod products .

[G7IVJ]

Thanks for all the replys.

The main transmission that is causing me problems is on 70.962.5 (or close to it!) on AM. It is Surrey fire brigade. They are talking about 60% of the time, and when they do, thats when I get between S3 and S9 of breakthrough on pretty much all freqs between 70 and 70.5 mhz FM and listening to them on a scanner I can hear it is definitely them causing me the problems.

As the FM1000 (and all x-pmrs) have a wide front end which as we all know is 66 to 88mhz, do u think it would help my situation if I were to obtain a purpose built radio for 4m - the only one I can think of is the AKD4001 or do you think I will still get the problem? Or maybe a HF radio with a transverter?

Thanks for the help.

[PA5DD]

I doubt another radio will solve the problem. What you need is a narrowband bandpass filter or a notch at the frequency you identified.

Problem is where to find such a filter, as most filters will be broader. Look for surpuls repeaters. I know that at least the STORNO CQF632 repeaters each have two good notch filters, that should be tunable to 70.9 MHz.

73 Uffe PA5DD

[GW8IZR]

more common in uk would be a CQF634D and a CQF9334D would also have a suitable filter in it.

As most of the CDA are now moving to Tetra you won't have long to wait for some peace and quiet - we are talking months not years.

Look out for duplex filter trays at the rallys made by AFL, sometimes they will have Home Office markings.

I have a seven section helical resonator as a receive pre selector on 70MHz and it has -60db stop band at band edges.

Regards Paul

[G8VOI]

Hi Simon,

I wonder how far you are away from the offending transmitter? Very surprised that it is directly causing you the problem unless you are perhaps within a mile or two of it.

If its that bad, another radio would certainly not cure it. I would have thought the FM1000 would be superior in any respect to the AKD. Although the front end is wide band, all stages are varicap tuned.

For comparison, in the Portsmouth area, the local fire brigade uses AM on 70.5875MHz. The transmitter is only a few miles from my QTH. The only problem I and a lot of locals suffer from is actually an intermod product that appears on 70.400MHz. The AM signal alone does not cause any problem, only when combined with another (as yet) unknown transmission. Given that there are several large towers close to each other, very difficult to find the second signal.

You might be able to find a decent notch filter to reduce the offending signal, probably your best bet until it QSY's.

73, Bob G8VOI

[G7IVJ]

I am located within 2 miles from the main TX source

So, any ideas where can I obtain a notch filter?

[GW8ASD]

I am located within 2 miles from the main TX source

So, any ideas where can I obtain a notch filter?

This company was mentioned on a previous thread.

http://www.parelectronics.com/amateur.htm

Good luck

Tony

[G3LVP]

[quote="G7IVJ"]I am located within 2 miles from the main TX source

What's a 'PA4ADN' conversion, it's not a front end pre-amp is it? I'd very surprised if an unmodified PMR RX suffered this much QRM at a distance of 2 miles. It might be worth while finding someone with a spectrum analyser to have a look at the offending signals to make sure that there's not a problem at the TX site, it's not unknown for PMR base stations TX's to develop faults. Alternatively try putting some attenuation in the feed to your RX and see whether this makes any difference, if it doesn't that would be a clue that there's a problem at the TX site.

73...

Ken

G3LVP

[G1HBE]

I agree with LVP. We had a similar problem here in Manchester a few years ago. The whole of 4m would suddenly erupt into beeps, distorted voices and buzzing noises. I made a phone call to the Newton le Willows RA site, who showed great interest. They booked a 'station inspection' at my home address (the only way to cover it) and took a listen to my 4m RX. Up came the interference and on went their speccy analyzer. They were horrified to see the entire band filled with s9+ of sh!t.
I'd already worked out that it only happened when a certain fire service TX came on air from the Werneth Low site, so they made a quick phone call and got it shut down THAT INSTANT. It turned out the tx had already been declared duff, but had been put back into service.

[G7IVJ]

Thanks all for the help and advice

A few weeks ago I ordered a Wouxun KG-699E handheld for 4metres - and today I received the order for the correct atenna connector - so I have plugged the handheld into the Sirio CX-4-68 antenna and my noise problem is much MUCH worse on the handheld. At least S9 across all of the 4m band, and extremeley noisey - much MUCH louder than on the FM1200 completely unworkable on the handheld due to the breakthrough / noise problem

Also I picked up a duplexer made by Procom, see information here:

http://www.procom.dk/eng/Page_menu/Prod ... /DPF_4_8_S

On the left hand side of the duplexer it is labelled "RX 77.0625" and on the right hand side it is labelled "TX 87.0625"

So my question is, how an earth do I tune this to filter out anything other than 4m?

Looking forard with interest to the replies

Thanks for all the help...

73s Simon

[GW8ASD]

What sort of test equipment do you have Simon?
Sweep generator / analyser etc?

I'm not sure a duplexer will help.
How were you proposing to use/connect it?
It's intended for combining a TX and RX split by 10MHz into a single antenna with the main aim being to stop the TX getting into the RX.
What you really need is a bandpass filter which passes 70 - 70.5 or a notch filter on the frequency of the offending transmission but very little loss on 70 - 70.5MHz.

Cheers

Tony