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-   -   FM signal boosting (https://www.vintage-radio.net/forum/showthread.php?t=168704)

Linnovice 5th Jul 2020 9:46 am

FM signal boosting
 
This is not strictly a ‘vintage’ radio question but hopefully you may be able to guide me in the right direction.
I have a small collection of FM tuners that I enjoy listening to, mainly R3. They range from a Troughline to Linn Kudos and include a Stereofetic, Sansui and Trio. All in good condition having been thoroughly serviced, recapped, new valves (where necessary) and realigned.
They are fed from a five element rooftop external aerial and are only fed one at a time. ie. there are no splitters involved, I move the connection as necessary. This aerial is aimed at the Wrotham transmitter which is approximately 75 miles away (I live in Tiptree, Essex).
My problem is that the signal I receive seems to be a bit low. ie. on the Troughline the magic eye valve only just about moves when tuned. The Kudos only shows about 65% on its strength meter. None of them have any full deflection on the meters. There are no significant hills or tall buildings in the path so I’m assuming it’s just a distance thing. The reception is ‘ok’ but I can’t help but feel that it should be better.
Would anyone care to recommend a booster or perhaps, an alternative remedy please?
Mike

Ted Kendall 5th Jul 2020 10:26 am

Re: FM signal boosting
 
VHF propogation is line-of-sight for most purposes, so aerial height is the simplest way of getting more signal. That and accurate aiming, but I assume you've done that already.

paulsherwin 5th Jul 2020 10:56 am

Re: FM signal boosting
 
75 miles is a very big distance for FM, even with a good external aerial and a big transmitter. Isn't there a closer transmitter you can use?

Radio Wrangler 5th Jul 2020 11:06 am

Re: FM signal boosting
 
If you're only interested in signals from one site, then you could get more signal from a larger, directional aerial.

Is whatever you using pointed for best signal?

There is loss in coax cable, so is your run as short as possible. Is the cable in good condition? Water can go walkies down the braiding, tarnishing it. This can create a surprising amount of loss.

If the cable is OK and the antenna is a big as you can tolerate, then think in terms of a mast-head preamp. A good sensitive tuner isn't going to be much noiseier than a preamp, so putting a preamp down near the tuners is not going to help much. But not all tuners have state-of-the-art low noise figures, so a preamp even if near them would help.

But what gives the best advantage to a preamp, is getting it ahead of the cable loss.

There is a case of horses for courses. Some tuners are deliberately made to survive operation in high signal areas with lots of big signals whamming around. They will deliberately be less sensitive in order to optimise intermod and blocking performance. Some other tuners are optimised for long range use and have higher sensitivity at the expense of intermod etc.

The Tuner Information Center (American spelling) makes interesting reading, but be cautious, the contributors are a mixture of people with lab grade test and measurement facilities as well as people using ears and making comments about perceived 'speed', 'authority' etc. There are also contributors from areas needing extreme sensitivity and others needing extreme large signal handling.

As you know which camp you're in, you should be able to sort the wheat from the chaff.

Some more signal from a better aerial is always the best thing, if you can do it. Mast head preamps come second.

If you are interested in stations from other locations, the amateur radio suppliers do antenna rotators. A small one will do.

David

Herald1360 5th Jul 2020 11:25 am

Re: FM signal boosting
 
A terrain mapping plot (megalithia.com) suggests that if you are in central Tiptree you are line of site but in the "first fresnel zone" so there could be obstructions causing signal diffraction problems. It might be informative to try with your exact location info.

The suggestions of more height, more metal in the sky (though a stacked extra 5-element will only get you 3dB) or a masthead preamp are all valid. Also worth checking how good your coax download is and the integrity of all the connections, particularly the most awkward one to get at, at the aerial.

Station X 5th Jul 2020 11:31 am

Re: FM signal boosting
 
Can the OP get a signal from Manningtree? That's where my FM aerial is pointed at.

matherp 5th Jul 2020 11:34 am

Re: FM signal boosting
 
What about pointing at Tacolneston, 250KW transmitter across very flat land from you. There is also a small transmitter at Manningtree - just down the road

Nuvistor 5th Jul 2020 12:32 pm

Re: FM signal boosting
 
For information this is an excellent article about the “new” Wrotham transmitter dates 1982, still in use. It shows Tiptree as in a decent reception area.
Well I found it a good read.
https://ia800504.us.archive.org/5/it...13/1986_13.pdf

Linnovice 5th Jul 2020 1:56 pm

Re: FM signal boosting
 
Thank you all for your contributions.
The aerial is quite high up, about four feet above roof ridge. The cable is about three years old and in good condition. There is no indication of water ingress.
I like the idea of Mannintree, I wasn’t aware there was a mast there. I may well have a fiddle and try and rotate the aerial and see if it improves the signal.

paulsherwin 5th Jul 2020 2:10 pm

Re: FM signal boosting
 
You seem to be marginal for Manningtree too, though it's a lot closer. You may be able to do some experiments by attaching a simple indoor aerial to one of your more sensitive tuners.

http://tx.mb21.co.uk/mapsys/map.php?mapid=41

Paul Stenning 5th Jul 2020 2:13 pm

Re: FM signal boosting
 
Quote:

Originally Posted by paulsherwin (Post 1266781)
75 miles is a very big distance for FM, even with a good external aerial and a big transmitter. Isn't there a closer transmitter you can use?

Around 20 years ago when I lived in Hereford I enjoyed Saga Radio from Birmingham (even though I was well under the target age-range back then). This was transmitted from Sutton Coldfield, around 60 miles away with Birmingham and the Malvern Hills in between. According to Wikipedia the current station on that frequency (Smooth Radio) is only 11kw but I think Saga was higher, maybe around 50kw.

At that distance, with a 5 element roof aerial pointing in the appropriate direction and a 12db booster in the loft, I could get good clean mono most of the time (apart from heavy rain or thick fog) and reasonably clean stereo if conditions were favourable (which was not very often, and relied on the programme material to mask the slight hiss).

I think under the circumstances I was lucky it was that good for the power, distance and terrain.

This does suggest 75 miles with 250kw and favourable terrain could be a bit better, but probably not much. I doubt you will ever get good clean strong reception from that distance, and will have to accept some compromise if it is the best transmitter for your location.

Julesomega 5th Jul 2020 2:33 pm

Re: FM signal boosting
 
Are any of these valve tuners? If so the sensitivity will be very poor by transistor standards. In any case, clear stereo from Wrotham should be possible, it's the same distance as Sutton Coldfield is for me. Don't go looking for bigger aerials, there are none on the commercial market now and in any case you'd only get another dB or so. Until people stopped buying rooftop FM aerials there were still on the market the original design produced in the 50s when the band was 88-98MHz. At about 101MHz the gain was zero and above that the direction was reversed. If yours is a Triax you'll be fine with that.

There are plenty of general purpose pre-amps available covering Bands I to V. Choose one with plenty of outputs for all your tuners, and of medium gain. I'd go for a masthead type, even if you only use it indoors. There used to be bandpass types for Band II but valve front-ends won't be bothered by strong out-of-band signals.

Take a look at, say Grax UK

Linnovice 6th Jul 2020 6:31 pm

Re: FM signal boosting
 
Hi all, thanks again for the advice. I have today fitted an OXi-GOLD 6-way signal booster. This has had a dramatic increase in strength at the tuners. My Kudos has risen from 65% to 89% which is great. The Troughline (valves) though is a bit odd. It seems to be overloading as the sound has a lot of fuzzy distortions in the background. Not terrible but not a pleasant distraction. Can you have too much signal?

Also, and I appreciate to some this will be a daft question but, obviously the signal booster is amplifying the signal. As it is at the bottom of the aerial cable is it amplfying warts and all or just the pure element of the signal? The sound output is very good and I'm more than happy with the result but just curious about the theory?

I know, I know. I do have a few lives left though ;D

Scimitar 6th Jul 2020 7:19 pm

Re: FM signal boosting
 
Yes, you can have overload and yes, all you are doing is amplifying signal and the noise, then adding the noise of the amplifier. So now your signal to noise ratio is worse than before. That's why you were advised to put it before the feeder.

Radio Wrangler 6th Jul 2020 7:26 pm

Re: FM signal boosting
 
Some tuners leak some of their LO out of the RF input, and if your distribution system has other tuners running you can get interference and funny swirly noises if you have more than one tuner on at once. Most splitters (including those packaged with preamps etc leak any signal coming backwards into them out their other ports.

The troughline has a damned great resonator right up the middle of it, and LO gets everywhere.

David

Linnovice 6th Jul 2020 7:41 pm

Re: FM signal boosting
 
Ah, a perfect description, funny swirly noises. So should I split off for the Troughline before going into the signal booster?

Excuse my ignorance. What is LO?

paulsherwin 6th Jul 2020 8:35 pm

Re: FM signal boosting
 
Local Oscillator. Standard superhet stuff.

I do think you are pushing your luck here. You are *never* going to get decent noise free stereo reception from a transmitter 75 miles away.

Paul Stenning 6th Jul 2020 9:10 pm

Re: FM signal boosting
 
You should be aiming for quality not quantity. Just trying to get enough signal level to make the meter swing across or close the magic eye by amplifying is unlikely to help unless the tuners are insensitive. But even then the signal level alone is not going to get you good noise-free reception.

You need a reasonable signal-to-noise ratio at the aerial before you start amplifying and distributing otherwise you are just amplifying the noise and adding amplifier noise.

75 miles from the transmitter is too far to get an adequate signal-to-noise ratio for good clean stereo reception, so whatever you do will be a compromise. It may well be that less amplifying means less added noise and actually cleaner reception even though the signal level indicator doesn't do much.

Normally amplifying is to compensate for distribution and cable losses rather than strengthening a weak signal. The more ways you split a signal (without amplification) the less of the signal each receiver gets.

It would be worth trying the other transmitters in the area that others have suggested and see if either of them is better. Otherwise, short of moving house, there probably isn't a lot more you can do.

Radio Wrangler 6th Jul 2020 10:32 pm

Re: FM signal boosting
 
You don't just get signal from an aerial, you get thermal noise created by the resistance of the elements and their temperature as well as thermal noise from the world around it. The effective temperature decreases if you aim above the horizon. Space is cooler than the earth. with the notable exception of stars! It was someone noticing this on a microwave link that led to the discovery of the 3 Kelvin background radiation anf thence the big bang theory. So if you have a very low signal and you stick in even a good amplifier every dB of gain you give to the signal, you give the same to the noise.

The preamp pays its way if your tuner has a poor noise figure, because then a low noise preamp can lift the signal and although it also lifts the incoming noise, the tuner was dominant, so you can get a net gain in signal to noise ratio.

Cable loss adds to the noise figure of whatever gear follows it. So mast-head preamps with a fair amount of gain can take out this contribution. Once you lose on noise figure, you've lost it forever.

Most people avoid studying noise because no-one likes it, but it's a case of know thine enemy and you can if not win, at least lose by less.

David

Linnovice 6th Jul 2020 10:33 pm

Re: FM signal boosting
 
Thanks Paul, I do take your point but it does leave me with a couple of problems. 1. My ladders (and my legs) are not long enough to comfortably reach the aerial mast to allow me to turn it and 2. We only moved in a couple of years ago and I swore to my better half that at 72 there was no way I’d be doing it again ��.

Seriously, I recorded the R3 concert this evening using my Kudos tuner with nothing else connected to the booster. The quality is extremely good so perhaps that is the answer. I can’t say hand on heart that I have ever been overly impressed with the Troughline. I’ve always likened it to what my mum would say about the radiogram in the fifties. “It’s a smashing bit of furniture and has a lovely tone”.

I will try some of the alternative transmitters as has been suggested though. Thanks again all.


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