VHF 405 Causing Interference on UHF 625 - any suggestions?

[DoctorWho]

I have a cabled system throughout the house, there are approximately 24 sets connected throughout the house via distribution amps and co-ax outlets. The UHF and VHF signals are combined by a distribution unit and then sent all around the house. Even thought this is purely a cabled system 405 can be picked up indoors on a set with a set top aerial! Amusingly the 7 modern colour sets also connected up are also being fed with a VHF signal as well as UHF, sadly though even those with VHF tuners do not tuen down low enough to pick up even the audio.

The one and only problem with the whole system is that, when 405 is switched on, it causes interference on UHF channels, any ideas how this can be overcome as, ideally, I would like to have 405 on all of the time? I have tried just using a professional UHF/VHF unit which combines VHF and UHF and sends the signals to a certain number of sets, everything is fine until VHF is switched on. I believe this may be harmonics? I have tried attenuation on 405, but this simply reduces the 405 signal and doe snot help with UHF viewing when 405 is on.

Any suggestions greatly appreciated.

Peter.

[paulsherwin]

I'm certainly no expert in this area, but I would think that either the VHF modulator is producing a dirty harmonic-laden signal, or the signal strength of the VHF modulator is overloading the UHF tuners.

In either case, reducing the modulator output with an attenuator may improve matters.

Good luck, Paul

[tubesrule]

Peter,
The output from the modulator on the lower bands is fairly rich in harmonics. This means that when outputting at 45MHz, there will be decreasing harmonics at 90MHz, 135MHz, etc. Also, depending on the video source, these harmonics can be up to +/-4MHz wide (System A modulator). It is quite probably that the upper harmonics, even though they are greatly attenuated are still interfering with UHF. Because the modulator is capable of generating frequencies between 30MHz and almost 900MHz, it is not practical to filter out these harmonics, and they make no difference when normally connecting to a set.

These harmonics are why I say in the manual you should not under any circumstances amplify the output of the modulator for RF transmission as the harmonics will also be broadcast.

That said, both issues, UHF interference and brodcasting harmonics can be solved with a simple, passive, in-line filter. These are sometimes available pre-made, or one can be constructed with only a few components. (capacitors and inductors) Depending on the highest frequency desired out of the modulator, and the lowest UHF frequency you receive, the filter cuttoff does not need to be very sharp.

Darryl

[oldeurope]

Good morning,
this is why I have a filter at the output of my modulator.
2 x 45MHz is 90MHz and this is UKW FM. (!)
I would never recommend to use a modulator without output filtering.

Kind regards,
Darius

[ppppenguin]

Darryl and Darius are both absolutely right. The output of an Aurora has a LOT of harmonics. They are good and strong right up to the 500MHz limit of my spectrum analyser so they will (not may) cause UHF interference if you distribute them in a combined VHF/UHF system. If one of the harmonics happens to coincide with a wanted UHF channel you've got problems. Worse than that, some of this interference may be finding its way back to the UHF aerial and causing interference on other people's sets.

A low pass filter at the modulator output is essential for your application. Should be readily available from aerial suppliers.

Here's a quote from my review of the Aurora

The output has a lot of harmonics. On channel 1 the 3rd harmonic is 11dB down while the 5th is 18dB down with further harmonics visible all the way up to the 500MHz limit of my spectrum analyser. There is also a low frequency spurious on the modulator output around 1.3MHz, about 27dB below peak vision carrier. This should not cause any trouble but a very simple high pass filter would remove it. The harmonics don’t matter for most users but you really must respect the warning in the manual and not connect the modulator output to an antenna as you could cause significant interference to non-vintage radio and TV reception. If you intend to use multiple modulators in an ambitious vintage system you will need to use decent filters before combining their outputs.

I hadn't anticipated anyone connecting an Aurora to a combined UHF/VHF system but the same rules apply.

[DoctorWho]

Many thanks for the input guys, and for your advice Darryl, much appreciated.

I can see the need for a filter, can anyone suggest a suitable one available on-line? Perhaps from Maplins, RS, CPC etc?

Having a combined UHF/VHF system works well for me, it also allows use of any set, be it 405 or 625, anywhere in the house, so if a way of filtering out harmonics were possible that would be great. The lowest UHF channel we personally use is 21 and the higest is 69.

Thanks again,

Peter.

[Sean Williams]

See my comments posted here.....

https://www.vintage-radio.net/forum/...ad.php?t=11913

Cheers
Sean

[ppppenguin]

A VHF/UHF diplexer may solve the problem.

Something like this
http://www.tvaerials.com/product.aspx?productid=581
On this page
http://www.tvaerials.com/shop.aspx?categoryid=121

may well work. Though something like this may be better

http://www.tvaerials.com/product.aspx?productid=371

A possible problem is that these are often designed for Band II on the VHF port so the response at Band I channels is unknown.

[oldeurope]

Originally Posted by ppppenguin

A VHF/UHF diplexer may solve the problem.

A possible problem is that these are often designed for Band II on the VHF port so the response at Band I channels is unknown.

Right Jeffrey, because of the return path most modern antenna parts work from 47MHz upwards.
So it will reduce the sound level and resolution at ch1.

Darius

[tubesrule]

Originally Posted by oldeurope

Good morning,
this is why I have a filter at the output of my modulator.
2 x 45MHz is 90MHz and this is UKW FM. (!)
I would never recommend to use a modulator without output filtering.

Kind regards,
Darius

Hi Darius,
Your modulator has the best output filter I have ever seen on such a device. Not only does it eliminate harmonics, but it is a proper vestigal side band filter! I don't think I have ever seen this on a modulator before.

This is much easier to do however when targeting a single channel. With a multi-channel modulator there is no easy way to provide this filtering, and no real reason to provide it. When connecting the modulator to a television, or several televisions the harmonics are meaningless. It is only in the cases mentioned here, broadcasting and mixing, that they become a problem which, while being valid uses, are not the norm and easily fixed with a filter.

Darryl

[DoctorWho]

Thanks for the URL's for the combiners, the only problem I see there is that they seem to be Band II and UHF, not Band I, would thgese work?

What would be involved in building a filter along the lines of the one you have designed Darius? I have tried attenuation but this kills the VHF signal far too much before removing the interference.

The aerial distribuition unit doesn't have a variable gain, but it's a good quality unit, when 405 is switched off it gives a perfect UHF signal to each modern set, similarly when 405 is on it gives perfect picture and sound to the 405 sets, the problem lies with the interference created on UHF when 405 is in use.

Any other suggestions appreciated.

Peter.

[oldeurope]

Hi, I can make a filter (4 pole) that attenuates everything above Birmingham.

Darius

[DoctorWho]

That's great Darius, could this be made to cut out the interference from Channel 1 (45 mc/s and 41.5 mc/s) ?

[oldeurope]

Originally Posted by DoctorWho

That's great Darius, could this be made to cut out the interference from Channel 1 (45 mc/s and 41.5 mc/s) ?

Yes, It will be a low pass filter.

Darius

[DoctorWho]

Thanks Darius, that would be fantastic, I'll send you a PM.

Peter.

[oldeurope]

Hi,
I tested a low pass (5 pole). The frequency response is excellent
Higher frequencies than 65MHz are attenuated. Definately enough attenuation at band III and UHF, not enough for UKW.

Kind regards,
Darius

[DoctorWho]

Thanks Darryl, that sounds very useful. Please excuse my ignorance, but what is UKW?

All the best,

Peter.

[ppppenguin]

Originally Posted by DoctorWho

Please excuse my ignorance, but what is UKW?

UKW = Ultra Kurz Waven (please excuse my German) = Ultra Short Waves

In English we say VHF, usually meaning Band II. If you look at any German radio, the VHF/FM band will be marked U or UKW.

So the proposed filter would not be adequate to stop harmonics that fall in Band II. Fortunately the lowest harmonic that can cause trouble is 41.5MHz * 3 = 124.5MHz, well above Band II.

[mickjjo]

Hi Peter, UKW = The standard FM band, 88-108 Mhz Band 11.

Regards, Mick.

[DoctorWho]

Thanks Mick, I've never heard of it referred to as UKW before. What does UKW stand for out fo interest?

All the best,

Peter.