Spurious emissions from Active TV antennas

[steve_cronin]

Flipping through Mr Heikell's excellent Greek Waters Pilot (version 9 has been around for over a year but Amazon tried to sell my wife v8) I came across a paragraph relating to "certain TV aerials emitting signals at such a frequency that they had been found to interfere with GPS sets." (or WTE)

Does anyone have any details as to which particular antennas he was talking about?

Steve Cronin

 

[Ships_Cat]

The US Coast Guard issued the Safety Alert at http://www.uscg.mil/hq/gm/moa/docs/11-02.htm for the USA ones. The Shakespeare antennas, which are the only good ones in the alert, only had a problem for ones made in a particular short period (and quite some years ago now). I have used the Shakespeare TV antennas on a number of boats (including my own) and never had a problem. On my own boat the TV antenna is mounted within a foot or two of the GPS antenna.

As to other problem active TV antennas which may be available in Europe I do not know any specific models, but I think the Safety Alert encourages one to purchase one made specifically for the marine market. Putting active radio frequency equipment on boats from places such as Tandy, Radio Shack and recreational vehicle outlets, or their European equivalents, is not something I would contemplate myself.

John

 

[scarlett]

People have TV on while sailing? This must be one for the "Lounge", along the lines of a cracker riddle, eg 'What do you call a sailor, who sails with the TV on?'

 

[Ships_Cat]

/forums/images/graemlins/smile.gif Strictly for marina sailors.

Actually the TV doesn't have to be on for the problem to exist. The active antennas usually have power on them all the time regardless of whether the TV is on or not.

I guess one could turn the antenna off whenever sailing but they draw very little and we normally don't bother.

John

 

[Birdseye]

[ QUOTE ]
Putting active radio frequency equipment on boats from places such as Tandy, Radio Shack and recreational vehicle outlets, or their European equivalents, is not something I would contemplate myself.

John

[/ QUOTE ]

Is an active TV aerial any more than an aerial with a small built in amplifier? Or are these active aerials something like a masthead TV set with the display down below? If the former I cant see where the problem could come from unless they are talking of malfunctioning equipment.

Its an interesting thought about non marine equipment, but I very much doubt whether marine electronics is any different in terms of internal design than any other electronics with respect to this problem ie marine kit makers dont operate to a different electromagnetic interference spec as far as I know. No consumer electronics should emit the sort of RF that would cause a problem to a GPS 2000 m away - after all GPS sets are used in many other applications than just marine. What the CG are highlighting is the problem of crap equipment, not the use of non marine equipment.

P.S. Just thought of how this problem could be arising. Do these things work simply by receiveing amplifying and then re-radiating the TV signal ie without a direct coax feed to the TV?

 

[Ships_Cat]

No, they just have the amplifier at the antenna. They are capable of interfering and that especially so with GPS due to the low signal strength of GPS (around –160dBw) at the earth's surface. I have not explored the mechanism as to exactly what leads to them interfering, but Shakespeare have been completely upfront stating this has been so with a limited batch of their antennas - an unlikely admission if interference were not possible.

I would agree that the 2,000 feet (feet, not metres) example seems a little extreme.

But I would not go along with there being no difference between marine (by marine I mean targeted specifically at marine use and exclude the land consumer stuff that is often pushed for marine use) and land consumer. Some of the land stuff sold for recreational use is absolute crap whereas even the bottom end marine stuff eg Shakespeare is of reasonable design and construction. Others like Naval are used on the high end vessels with no quibbles about quality. I assume that Glomex make probably one of the more common lower end TV antennas targeted at marine use in Europe but while I have used their passive radio antennas I have no experience with their TV antennas.

I think the other thing with antennas targeted specifically at the marine market is that if there is a fault that affects other marine equipment it will come to light early on and be fixed. I don't think that is so with the land recreational stuff - for example, Shakespeares problem only affected a small run of antennas before it was recognised and fixed (I think before the USCG came to issue their alert). I doubt that was the case with the Tandy, etc ones. I have not looked at the Shakespeare internet site lately but until at least recently they actually carried a warning about which of their antennas had the problem of interfering with GPS - I doubt whether Tandy, etc would have.

It has sometimes passed through my mind that the forumites that report loss of GPS and seeking blame on other things or asking for reasons perhaps should look at their own boats first for the cause.

John

 

[scarlett]

It happened to me three times when passing within a couple of miles of an Italian naval base on their west coast. I don't know the purpose and it was in the SA days too.

 

[William_H]

GPS and to a lesser degree VHF, navtex etc being recieving devices are susceptible to interference from some electronic devices. I don't think you should worry but be aware if your GPS is not functioning correctly it could be interference. You should be able to remove power from all electronic devices one at a time to confirm or refute interference and of course be aware of passenger's electronic toys.
Here is the rub with airlines, interference is highly unlikely from devices but in a 400 seat airplane all sorts of toys can be used without the crew knowing so they can not eliminate them if a problem of interference appears, so they ban them altogether. In your own boat (or aeroplane ) you can control the odd electronic devices so turm them off if you have a problem.
I find it highly surprising an active TV aerial can cause a problem but then all things seem possible, Just be aware. olewill

 

[Ships_Cat]

I find it highly surprising an active TV aerial can cause a problem

Hi Will, I found it surprising too. I don't know what the amplifier configuration is in them so this is just a guess. I assume that in the faulty ones the amplifier, especially being required to be a wide bandwidth device, may be too non linear over part (or all) of its bandwidth and so produce harmonics of the received signal. These then reradiated by the antenna itself and interfering with the GPS.

Have no idea if that is the actual reason for the problem ones, but it does raise the possibility that interference will only occur when a TV (or any other rf signal is in the amplifier's pass band) is being received and of a frequency producing the right harmonic. It may be that the field strength of that received signal has to exceed a certain amount too.

If so, that means that a simple test for interference does not exist. The problem will only show itself when in the right rf environment. May also explain why some report problems in areas where there are potentially high rf fields - I wonder what else there is on board that could do the same reradiation of harmonics thing (VHF radio turned on, SSB turned on, domestic radio and TV set???. Although one would expect VHF and SSB at least to have good quality pass band filtering on the input side that would block any higher order harmonics).

Perhaps I am just dreaming? I would certainly be interested in any feedback.

Regards and best for the New Year

John

 

[William_H]

Hi John that does sound like a plausable explanation of interference from a TV antenna amplifier. However you would imagine we are talking pretty strong field strength of TV or similar possibly even VHF com.
I can't imagine the whole thing will be a real worry it might knock out the GPS but wouldn't give errors. Happy New Year to you too. olewill

 

[Birdseye]

Your post got me wondering so I did a bit of digging about interference specs. The following is cut and pasted from the Offcom site. GPS frequencies are about 1.2GHz.

"There is already considerable use of radio communications above 1 GHz (GSM 1800, 3G, radars, Wireless LAN, etc). There are only generalized maximum limits for spurious emissions from radio transmitters and many radio standards and ITU recommendations allow spurious and out-of-band emissions above 1 GHz at fairly high levels.

There are currently no agreed standards for unintentional electromagnetic (EM) emissions from ITE equipment above 1 GHz in Europe, although suggested limits are being put forward both within ETSI and CISPR. Preliminary measurements by Ofcom’s research laboratories have demonstrated that there is increasing EM noise produced by modern non-radio devices (PCs, pdas etc) at frequencies above 1GHz. "

So in other words, many bits of kit on board may well radiate spurious signals strong enough to blot out the weak GPS one. Indeed I have heard of mobile phones doing just that.

Still doubt your point about marine equipment. Ignoring the practicality of buying just specific marine kit (is there such a thing as a marine moby, or TV?) marinising is likely to be more about laminated boards and good waterproofing than it is to be about spurious emissions. I would have expected marine kit, if it differed at all, to be worse on emissions since there rarely is anyone else out there to complain. Not like blotting out your neighbours TV with your ham gear!

Interesting post.

 

[roger]

interference from military comms aerials

This is a bit off topic but my NASA depth sounder gave completely daft readings when I was passing close to a military ( I presume ) lot of aerials on the coast of Norway. Now the transponder of the depth gauge was mounted in oil inside a rather thick hull and usually gave up at about 30 metres anyway but in here it just gave random numbers. This was a mite worrying as there were rocks about the place.
It may bell be that the electronics in the sounder had to give a very high amplification because the usual return signal was so poor that it picked up the military signals. I've since then fitted a Silva gauge which gives up at 180 metres and is a lot more useful.

 

[Ships_Cat]

Hi Birdseye - my point in respect to the active TV antennas specifically targeted at marine use is that if there is a problem associated with marine use then it is likely to be identified and addressed. Otherwise, as in the case of Shakespeare, their whole active antenna market would have dried up. I am not saying that in general "marine" equipment such as "marine" versions of car radios, TV's or whatever are necessarily better (in fact I always specify normal consumer equipment for on board HiFi, TV sets, etc).

On the Shakespeare site at http://www.shakespeare-marine.com/antennas/tv/safetyalert-tvantennas.htm they are still running the warning about which of their antennas were faulty. They have clearly identified the problem and fixed it and I suspect other manufacturers targeting the marine market have themselves taken notice. I suspect that manufacturers of active antennas whose main market is recreational vehicles and general land based users are disinterested or not even aware of the difficulty.

In my experience the active TV antennas aimed at the land recreational market are generally junk. Perhaps that can be shown by the number of posts there have been from forumites (mainly on the MoBo forum) as to the poor performance of active TV antennas. However, the Shakespeare and Naval ones do work, obviously not as well as a gain antenna and with lesser performance at the lower frequency (50 MHz) where the antenna may be small in terms of wavelength but they do work. As said, I cannot comment on Glomex, which is possibly the more common low end active antenna targeted at marine in Europe.

John

 

[Gypsy]

John, your hypothises may be valid. Another possiblilty is that since these "pre-amps" are required to be high gain and wide bandwidth it is a classic opportunity for them to "take-off" or "honk" or any other term to describe them oscillating and therefore becoming a transmitter themselves. The frequency of the oscillation does not necessarily have to be in the main bandwidth they are designed to work and of course there are the harmonics which could easily get into the 1.2gHz area. To be any more informed one really needs a spectrum analyser to see just what IS in the airwaves from external or internal devices but that is highly unlikely in the non-commercial boaty environment but I would imagine the comms guys on the big Navy ships could tell us a thing or two if they were only allowed to discuss such matters.

Ray