TVI / Front end overload

[VK3LDR]

Tried a torroid and no luck, same interference. The point about the mast sharing tv and 2m co-ax was something I hadn't given any thought... I'll see how I go tomorrow relocating the aerial & mast. The close proximity of TV and other co-ax cables is a bit unavoidable. The "shack" (one of 2 spare bedrooms) is next to the main entrance of the house, so 2 brickwalls away and there's the TV... The TV antenna co-ax run down the wall is in the cavity on the lounge room side... and that is the where the TV is.... We do have 1 other TV, which does get interference as well, and that was also on bunny ears. That's now in the main bedroom, so I might test that out tomorrow during the day to see about TVI on that TV as well in regards to where it is.

One of my resolves might be to relocate the TV antenna closer to the TV and AWAY from the 2m aerial. Well, the logistics are getting complicated, I'll keep you all posted, and I haven't forgotten about getting the pics up either, I'll get to that too probably tomorrow (I don't have a memory card reader or cable for the PC so it means transferring files around the place via a laptop etc. etc.)

cheers,
Dave

[VK2OMD]

Tried a torroid and no luck, same interference.

That addresses another ingress mechanism, and there are many (eg mains, inside the coax, outside the coax, AV connections etc).

You are stabbing in the dark, you need someone experienced and competent on-site to help you to systematically review the health of existing systems, and determine the ingress mechanisms at play with a view to finding a solution... and it might not be just one change.

Sometimes, not knowing enough to solve the problem means not knowing enough to describe the problem fully, or putting it another way, knowing enough to describe the problem fully is knowing enough to solve the problem. An experienced person's eyes might see things that you haven't recognised as relevant and aren't describing here.

Owen

[VK3AFW]

Dave,

You are not very far from me. I have a TVI filter that I loan out to neighbours and friends to see if it cures the problems they experience. It is effective on HF and 2 m. I will drop it in to your letterbox (ur # 690, GH Rd pse confirm) tomorrow AM. I want to get it back but it will certainly confirm whether the problem is RF down the inside of the coax. It incorporates a braid breaker so it gives some immunity to RF down the outside of the coax. Fixing the TV aerial is of course a good thing to do regardless as if there is corrosion or poor connections of some other form then all kinds of inerference can occur.

If the filter is effective then we can look more closely at yours and get it working.

73
Ron,

VK3AFW

[VK3LDR]

Hi there,

Cheers Ron for the offer on the filter but probably no longer required actually. I just spent the last 2 hours re-erecting the aerial and it's now about 9m up and secured with guy wires. It's now about 2m higher up in the air than the TV aerial and it's about 3m away from it and guess what, No more TVI from what we could see on Channel 9 or 10, no cut outs on the STB on those channels either. The only issue is Channel 7, there is interference still and the STB only has errors when you initially key up the mic... after that it's ok. Note though that 7 has poor reception, 10 moderate and 9 is clear, so that explains why 7 is so bad. I think as has been suggested earlier, time to piff out the 'ol TV aerial and put up a new one. Probably still a combination aerial though.

That listed address is not my home address, it's really just a postal address and it's actually an office belonging to my parents and brothers business. I'm actually in Boronia, considering whether or not to bother getting a PO locally (and btw I work in the city on Bourke Street)

I did take some photos and downloaded the earlier ones finally onto a USB stick but the latest ones are still on the SD card and now I don't have time this arvo to get them sorted but might do it while at the inlaws on the laptop and upload them then! On that note I think they're waiting for me to pack up all of my tools and get washed up... got some shopping to do...


All of your input into this topic has been greatly appreciated, but it appears the positioning of the duo band vertical was the problem all along primarily and of course the fact that TV reception is a little on the weak side.

[VK3AFW]

Hi Dave,

Well it seems you are pretty much out of the woods. Well done. If you double the distance between the Tx and Rx antenna you will get about 1/4 of the signal in the Rx. The inexactness comes about because the near field of a transmitting antenna behaves differently to the far field and there are nearby bits of metal, house wiring, roofing, valley gutters, rain gutters, etc that will distort the field.

Thanks for correcting me on the address! Boronia does have some pockets of poor reception - the TV radiation pattern is not ideal and even when the towers are visible the signal isn't always rock solid. A new antenna pointed at the channel 7 tower is a good idea if the old one is showing some corrosion etc.. Check the ACMA website for where the tower is - Melways also will also help. I have a vague recollection that the analog and digital services may not be co-located for each channel. VK1OD might comment.

A combination antenna is OK but don't get the smallest one as you could do with the extra gain by the look of your strength nunbers. Seperate VHf and UHF antennas would be better providing you use a proper combiner.

New coax is highly recommended. Make sure the rain can't get into the coax at the antenna end. Cover with silastic if you don't get a good enclosed balun.

There is very cheap coax and moderate cost, multishielded coax. The cheap coax has as little copper, espically in the braid as the maker can get away with. Best to spend the extra money and take the extra trouble with the connectors to get lowest loss and best screening.

Good luck.

73
Ron,
VK3AFW

[VK2OMD]

... I have a vague recollection that the analog and digital services may not be co-located for each channel. VK1OD might comment.
...

Ron, I don't know in the specific case, but that would be good information for Dave to run to ground, and find out if he is looking at a lower powered translator or a high power tx.

I understand that quite a number of the DTV transmitters are operating on an interim restriction of 25% power to minimise risk of interference to analogue TV, and that they will increase when analogue TV closes.

By way of a comparison, I am about 5km from the main Canberra TV transmitters on Black Mountain, and I cannot disrupt the any DTV channel with 100W on 2m (FM or SSB) and the worst case antenna is only a few metres from the TV antenna. I do have a 12dB F attenuator right on the STB connector, it was installed to remove occasional interference on a couple of channels with >50W. The STB is a cheap Woolies number.

Dave, forget the rabbits ears. If an RI visits someone complaining of TVI and they are using rabbits ears, problem is solved... get a decent antenna. If rabbits ears work better than the outside antenna, the outside antenna needs fixing.

Owen

[VK3LDR]

I really appreciate the input and feedback on this matter from all of you. I'll upload a few photos later or just send a link to my page with the updates antenna install pics. Originally I was going to mount the duoband vertical over the top of the TV antenna, well not exactly over the top but about 50 cm away but just as high. The guy who helped me install (John, VK3TVZ) suggested the centre of the roof for preferrable guy wire securing points which has worked out ok.

I know this topic digresses too!


I'll shop around for TV antennas I think, my wife doesn't seem to care about the reception because the STB picture is clean compare to analoge. I think improved CH7 reception will kill off all TVI though which for me means more work on the TV reception side of things. Correct in stating Boronia is a bit odd too, I am in a slight dip. Ron, FYI, I grew up in Caulfield & used to live near Caulfield racecourse. I have yet to retrieve the trusty old 27MHz Itron from my parents place as a matter of fact!

I'm now going to use the same mast to erect probably a multi element or a multi band trapped dipole for 80/40/10, the current 40m up a tree works fine but I'm thinking of turning that into a portable dipole. Besides, less co-ax runs are easier to manage, I don't have enough left over rg213 though so I'll probably use up the rg58 I have. And... I digress again.

73's all,
I'll post up some links and the pics of the filter I assembled later.
Regards to all,
Dave
VK3FDMR

PS. The TVI testing was done at full power as well of course.

[VK3LDR]

Here's 3 pics.. 1 completed job shot, 1 "action shot" just for fun and the final is a pic of the finished band stop filter..





A link to an album of general aerial setups and work http://picasaweb.google.com/roity57/Ama ... ennaSetups

Cheers again for all the input,
Dave
VK3FDMR

[VK3AFW]

Hi Dave,
All good stuff. One thing you might try is reducing the coil to 4 or even 3 turns. If the minimum c of the capacitor is a little bit high you might find it hard to hit 2 m. Tuning can be very sharp. I use a sig gen and milliwatt meter for setting up filters.

73

Ron
VK3AFW

[VK2OMD]

I do have a 12dB F attenuator right on the STB connector...

Dave,

A short word on the way in which an attenuator works, it has benefits beyond the obvious.

Noting that as far as the desired TV signal is concerned your transmitter is not in-band, but your transmitter does transmit some level of wideband noise that may be in-band to the TV signal.

Inserting an attenuator of say 10dB reduces both the TV signal and your transmitter signal (including its wideband noise) by 10dB. That might not seem much advantage, because it hasn't improved the S/N ratio for the TV signal, in fact it has probably degraded it somewhat.

Another mechanism for interference is more complex, new in-band interfering signals can be generated by mixing of external out-of-band signals in the TV receiver. This mixing is due to non-linearity, usually mainly in the first stage of the TV receiver, and is also known as Inter Modulation Distortion or IMD.

Now, lets consider the affect of a 10dB input attenuator on IMD products.

The easiest to explain are second order mixing products (f1-f2, f1+f2). Since you have attenuated both input signals by 10dB, the second order products are reduced by 20dB. Although you have reduced the TV signal 10dB, you have reduced the 2nd order IMD interfering signals by 20dB and so potentially improved the S/N.

A more interesting and important mix are third order mixing products. Since you have attenuated both input signals by 10dB, the third order products are reduced by 30dB. Although you have reduced the TV signal 10dB, you have reduced the 3nd order IMD interfering signals by 30dB and so potentially improved the S/N.

If you want a simple demonstration of this effect, try a ham handheld on an external antenna with and without a 10dB attenuator on the input (don't transmit into the attenuator), S/N may well increase with the input attenuator in circuit. A lot of ham mobiles will exhibit this effect to a somewhat lesser extent. Many ham mobiles are pretty useless in the big cities without the input attenuator in circuit.

Hams often refer to this effect as 'pager interference', but it is not restricted to paging transmitters, and the name is not a good one.

Owen

[VK3PY]

Hi Dave, Ron, Owen and others.

I've followed this discussion with interest. Most of my immediate neighbours and a few further afield have mast-head or distribution amplifiers. My transmissions on 6m, 2m and 70cm used to go through them like a dose of salts. That is until I fitted them traps for each band. Now I can run full legal power into high-gain yagis with total impunity (although there's always a new installation somewhere within range that gets clobbered!).

I have a couple of suggestions concerning the trap filter. I have had great success using a SERIES tuned circuit to ground across the TV coax. Six turns, 6mm dia. in series with a 10pF mylar trimmer does the trick. This configuration typically gives a 30 dB notch at 2m.

To tune the trap, I suggest placing it in line with your rig's antenna and use a local beacon or repeater as the signal source. Tune the trap for minimum received signal. This is easier to do than Ron's suggestion of tuning for minimum TVI while transmitting. With such a deep notch, tuning will be critical - do it carefully.

Incidentally, one worthwhile benefit of having 148 MHz pagers is that many manufacturers of TV mast-head pre-amps fit them with a 148MHz trap as standard practice. These usually take the form of series tuned trap to ground consisting of an air-wound coil in series with a disc ceramic capacitor soldered across the pre-amp's input. They tune it by spreading or compressing the coil turns to resonate at 148 MHz, which gives adequate rejection of 2m signals.....unless the twit who installs it pushes the coil out of shape while terminating the coax, as I've seen on two occasions. The beauty was that in one case it took out Ch 7 completely - priceless!

Chas
VK3PY

[VK3LDR]

Thanks again for the input, you guys have been great in trying to help me out. I'll note that for reference too and maybe give that a go as well. I get the Olinda 175.075 MHz repeater full scale 60+ with the ic706 built attenuator on so that's probably an easy starting point... Just need to trigger it!

Cheers,
Dave

[VK2OMD]

... I get the Olinda 175.075 MHz repeater full scale 60+ with the ic706 built attenuator on so that's probably an easy starting point... Just need to trigger it!

This comment is probably relevant to another thread, but...

If you compare the S meter on FM and SSB you will probably find they read very differently. Typically, the FM S meter goes full scale on about 10uV, which is not a very strong signal, though plenty for full quieting.

Thing is, does inserting the attenuator improve S/N (ie make the recovered voice clearer, reduce the interference effects)? Try it with the preamp on and off.

Owen

[VK3AFW]

Hi All,
Further to Chas's comments, I have used two half wave coax stubs connected across the coax (a T connection) spaced a quarted wave apart. The free ends are left open so they reflect a short on the coax at their resonant frequency. It is the same principle as the shunt series tuned circuit. The stubs do need to be accurately cut to length. While they could be tuned using a repeater I prefer the sig gen & power meter approach. Instead of a sig gen the Tx could be used set to low power - 1 w or less - and a SWR meter connected to a dummy load usd to provide the null indication. The quarter wave spacing need not be exact and can be calculated from the nominal velocity factor of the coax, typically 66% of the 300/f value. The shunt stubs should be cut longer than calculated and adjusted one at a time. Start by cutting a few mm off and observing the indicated transmitted power. Snip some more off and repeat until the transmitted level starts to fall. Then cut 1 mm at a time until the signal drops to a minimum. If you go too far you can replace the stub and start again but this time you will know how low the indication can go before you cut off too much. Temporarily remove or short the tuned stub at the open end and trim the second one. Restore the first one. Attenuation exceeding 50 dB can be obtained. The coax can be coiled up and held in neat loops with cable ties. Now of course you know the real velocity factor of th coax and can make a second trap with more accurate quarter wave spacing. It is still desirable to trim the stubs to get the best attenuation. Advantage - cheap and effective, disadvantage, slightly fiddly to adjust and bigger than a couple of tuned circuits.

One stub alone should give 30 - 40 db attenuation (indicated voltage down a factor of 30 to 100) and two stubs 50 dB plus (voltage down by a factor of 300 plus)

The advantage of a trap or filter over an attenuator is that it only affects a narrow range of frequencies with minimal effect on the desired TV signals. An advantage of an attenuator, apart from being readily available in some shops, is that it reduces all signals and can dramatically reduce cross modulation effects as descrobed by Owen. Sometimes both are needed together.

If the Tx is used for tuning LC traps the power must be kept below 100 mW or else there might be some damage done to the capacitor.

73

Ron
VK3AFW