Quad FM3 poor reception

[ppppenguin]

I was listening to the last night's Prom on my main hifi system. Quad FM3, 33, 405 and ESL63. The piano sounded horrible and distorted. So what was wrong? Switching to mono made things rather better, and the hiss level dropped, so this was clearly a tuner/aerial problem.

I live in an area with good TV and VHF radio reception. Quite a long way from Crystal Palace and Wrotham but high up and clear path. With a decent UHF aerial on the roof I'm passively splitting it 4 ways and still have at least 12dB margin before Freeview goes bad. THe VHF aerial is one of those horrid "halo" things, less gain than a dipole, but with a strong signal that hardly matters. Both aerials are high on a chimney and are now over 20 years old. Downlead is old fashioned "low loss" co-ax. Some years ago I had the downleads swapped over and everything still worked OK except that (analogue) Channel 5 was bit noisy.

I wheeled the spectrum analyser (HP4195A) into the lounge to have a look. Didn't bother with a 75R to 50R minimum loss pad as it wouldn't make much difference. Radio 4 (93.5MHz) was about 130uV, R3 (91.3MHz) about 10dB below that and R2 (89.1MHz) about 10dB lower still. That aerial and/or its downlead are definitely not OK. Out of curiosity I looked at the TV aerial. My analyser only goes up to 500MHz so I could see TV multiplexes on Ch22 and Ch23, an odd signal at about 390MHz, good strong DAB around 225MHz and FM radio that while not strong was better than the VHF aerial and fairly equal on R2/3/4.

[ppppenguin]

Had a look in the stores and found an old Antiference Band II antenna. Opened it out, tightened the nuts. Old and a bit horrible but it should do for tests. It's not even got a balun, the co-ax is connected directly to the dipole.

[ppppenguin]

Rigged up that aerial to the spectrum analyser and just sitting on the table in the garden it's giving me about 350uV on R3 and R4. About 3dB lower on R2. Changing the direction and polarisation I can get up to 500uV or so.

I don't have access to the chimney so I'll try to find somewhere accessible for this aerial and find a cable route back to the lounge. Possibly a loft or outdoors above a flat roof.

[Mike Watterson]

A 75 cm vertical with 3 or 4 ground plane rods will work better than a halo for VHF-FM. I used to use one made up of 5 bits of coax in the loft with vertical hung from beam with plastic cord and the 4 ground plane bits lying on the joists/rafters.
I don't understand why they are still sold for VHF-FM radio. It's been a LONG time since that was only Horizontal polarised.

[JohnBS]

You ideally want about 1mV for good stereo reception. 300uV OK but you will hear some additional noise if the receiver has a low noise floor. I believe those halo antenna are omni so sub-optimum if you're tryng to improve signal and reduce multipath.

John

[ppppenguin]

John, 1mV is what I thought for good stereo.

The halo aerials are low gain, I've heard -2dB wrt dipole. But I'm in a good location so it should be adequate. Since the aerial looks OK and there's not much to go wrong then the downlead must be in trouble. Probably abraded badly at chimney level.

With a bit of effort I reckon I can get up there safely. It's accessible from a large flat-roofed dormer. Checked the ladders I have and they are long enough to get there safely.

In the meantime I'll patch the tuner across to the TV aerial. Or rig the 3 element Band II aerial somewhere I can readily connect a downlead.

[Mike Watterson]

Can be -6dB compared to a whip and a bit worse WRT dipole as a vertical dipole spaced off the pole has more gain than a 1/4 whip (Marconi monopole) with ground plane.
The ONLY reason for them is Horizontal Omni. So pointless for FM Radio nowadays.

[ppppenguin]

Had a look with binoculars. The connection box on the halo is rather fetching shade of green. Like a nicely aged copper roof. Heaven knows how a plastic box got that way. Can't see anything wrong with the downlead but even if it was worn halfway through I doubt I'd be able see it.

When I finally get up there, I'll replace the halo and both downleads.

PS: It might have been green all along: https://www.screwfix.com/p/labgear-omni-...-kit/52645

[Jac Janssen]

From the brochure:

   

Jac

[ppppenguin]

Thanks. So I really need at least 200uV and ideally 500uV or more.

It's possible that the tuner has developed a fault which is only now showing up the weak signal.