HELP! OCF dipole setting off neighbour's home alarm

[VK5IR]

Well, the thread title says it all.

I've had a half wave 40 meter dipole in the air for the last 4ish years. Never had a problem. A month ago I cut it in half to experiment. Worked well, never had a problem.

Today, I took it down and put up a 40-10 meter off centre-fed dipole. PROBLEM! A short while later I noticed one of my neighbour’s house alarms going off. Didn't think much of it.

Tonight, at 11.15pm, it went off twice. Just after I dropped a carrier on CW with 100 watts on 20 meters.

I had also dropped carriers on 40, 15 & 10 meters too a little earlier, but I had not noticed anything, although I do have 3 kids and they get pretty noisy at times, so if it did go off then, I wouldn't have noticed.

Here's the setup: Jetstream commercial 4:1 balun, rated at 1.5KW, no RF choke (although there was on my previous dipole via home brew RF choke balun), raised to roughly 6 meters height.

Elements are roughly 6.76 and 13.52 meters in length respectively.

The old dipoles were in the same position and at the same height.

Anyone got any ideas what my issue is and how to fix it?

Cheers

Theo.

[VK4TI]

With that sort of thing ask your self , grounding good ?, not some skinny wire but a real ground . could moving the angles alter the response , would a choke help ? , maybe some toroids over the coax for a start . Do you have anyway of checking resonance and actual rf disposition across the wire , is it current or voltage balun and would swapping to a 1:1 assist . Simple logical and methodical , whats the most obvious different from the old install and is it the same coax and are the plugs and fittings getting old
Good luck with it but you might find the psu and the wire connecting the wall wort to the alarm box is a simple nice wire antenna sucking your rf in and can be fixed with a cap or two ( sometimes it's the wires on the detectors decide be be difficult) in which case not your problem re fixing really . remember sort your station and don't take on what maybe a bad alarm install or a recent fault

[VK3MEG]

I would be fitting the rf choke and checking your grounding as stated vk4ti gave the best advice . good luck with it hope to work you in the john moyle

[VK2CBL]

I had exactly the same problem four years ago after coming back on air after a break of 20 years.

I would set off my burglar alarm with only 30 watts on HF.

Fitting ferrites on all the cabling only slightly reduced the problem.

Was not until I observed that the tamper function on the alarm was being triggered.

Fitting 0.047uf caps from each tamper line to ground (common line on alarm) completely solved the problem.

Now able to run full power on HF on any band without setting off the alarm.

Now all you have to do is to get your neighbour to agree to have the capacitors fitted.

[ZL1RS]

It could be due to the inherent radiation pattern differences between the old and new antennas which will probably be substantial because:

1) The old 40m dipole probably radiated mostly off the antenna wire with your home made choke preventing common mode current on the feed line and radiation from it.

2) The uneven leg lengths of the OCF antenna result in a high imbalance at the feed point which will cause high common mode current on the feed line and substantial radiation from it (see foot note). A 4:1 current balun can reduce this effect but according to their website, Jetstream's 4:1 balun (JTBAL41) is a voltage balun ... so there will likely be a large common mode current on your OCF antenna's coax feed line and substantial radiation from it as a result.

Replacing the 4:1 voltage balun with a 4:1 current balun might do the trick. Two ways of providing a 4:1 current balun are:

1) by a particular winding configuration on 2 toroids

2) or a 4:1 voltage balun followed immediately by an effective 1:1 choke.

Because you already have a voltage balun on the OCF antenna and a 1:1 choke from the old 40m dipole, it is probably easiest to try method 2).

BTW, common mode current on coax feed lines can be measured with a very simple device such as:
http://vk1od.net/measurement/icm/index.htm
I made one (not nearly so elegant) in a couple of hours ...

It has been quote revealing ... my 6m antenna system feed line has hardly any common mode current measurable, but the feed line on my 40 dipole (with a choke balun at the feed point) has substantial common mode current. (Now I "need" to find out why!)

73, Bob ZL1RS


Foot note: Some marketeers try to make good of the OCF antenna's large common mode feed line current and the resulting radiation from it by offering a "vertical radiator kit" for OCF antennas! It consists of a "special length" of coax hanging down from the OCF antenna's 4:1 voltage balun and a 1:1 choke at the transmitter end of the "special length" of coax. There is radiation off everything past the 1:1 choke (assuming the 1:1 choke itself is effective at the frequency of use).

[VK5IR]

I wanted to get an idea as to what band was setting off the alarm if not all of them, but when I tried to replicate the issue this morning I could not get the alarm to go off.

I'm not 100% sure which house it is exactly. I live on a corner block so I have houses all around me but I have narrowed it down to two houses, the one I suspect is an unoccupied brand new house which is for sale. I suspect that maybe the owners may have turned the alarm off. I did notice them around today (they come from time to time to do some cleaning), so when they leave I will give it a crack again.

As far as what has changed exactly, the only thing that has changed is the dipole antenna itself (balun and wire elements), that's it. Feedlines, connectors etc. are exactly the same.

As stated in my first post, I don't have an RF choke on it, but if problems persist I will try one. Will a few snap-on ferrites at the base of the balun do the job?

This short video will give you an idea of the setup and the houses around me: http://www.youtube.com/watch?v=R0DTOazU ... e=youtu.be

[VK4TI]

I would think maybe wind an ugly balun and tape it directly under the balun just onto the pole for a check , it should reduce feed-line radiation some , is there someone local with the gear to assist like an antenna analyser ? , change the balun if you can but having run a similar antenna ages ago although it worked great I ended using a 9:1 balun wound on some conduit or I wiped my own analog tv , something is obviously coupling into the alarm (duh) , it may well need some caps added which I suggest you treat with suspicion as some of the cheap alarms are just plain cheap .

[ZL1RS]

... I don't have an RF choke on it, but if problems persist I will try one. Will a few snap-on ferrites at the base of the balun do the job?

How effective the snap on ferrites are as a common mode choke will depend on their properties. Easy enough to try though ... when the problem surfaces again. The number of clip on ferrites to use will be dependent on their properties and how much power you are running. Passing the coax through a large diameter ferrite in 2 or 3 loops will increase the choking impedance more or less by the square of the number of turns and may be much more effective that just clipping the ferrite over the coax (effectively one turn) ... but watch for heating of the cores when power is applied.

FWIW I've been playing with a few ferrites with the view to checking their actual characteristics with a Vector Impedance Analyzer.

The upper example is a string of ten #43 mix cores over LMR400 coax. It has a common mode impedance around 800-1200 ohms on the upper HF bands, dropping off to "it might still do something on 80m" and pretty well useless on 160m ... but depending on the antenna it is connected to, should handle power up to the limit of the coax (still to be tested). The next down has 2 turns through each of three #31 cores and a common mode impedance between 1000-1900 ohms between 6MHz and 41MHz, and should have a moderate power handling capacity (still to be tested) due to the three cores and only 2 turns on each core. The next choke (4 turns on a #43 core) has a common mode impedance between 1000-2100 ohms from 5.5MHz to 29.5MHz but just a low power handling capability due to the one core. The lower example is current balun wound from RG-58 coax in "W1JR style" and has a modest common mode impedance (about 700 ohms) but that is fairly "flat" from 1.5MHz to 40MHz (unknown core from the junk box). But it surely will not handle much power with the core being so small (1.5" dia x 0.25" thick). There are flaws in my measurement process, but the results give a general idea ... but there is no substitute for trying "something" in real life.

Keep in mind that the burgler alarm is not meant to be an HF receiver, so the real problem lies there ..... but getting the alarm owner to see it that way may be another story

73

[VK4TI]

... I don't have an RF choke on it, but if problems persist I will try one. Will a few snap-on ferrites at the base of the balun do the job?

How effective the snap on ferrites are as a common mode choke will depend on their properties. Easy enough to try though ... when the problem surfaces again. The number of clip on ferrites to use will be dependent on their properties and how much power you are running. Passing the coax through a large diameter ferrite in 2 or 3 loops will increase the choking impedance more or less by the square of the number of turns and may be much more effective that just clipping the ferrite over the coax (effectively one turn) ... but watch for heating of the cores when power is applied.

FWIW I've been playing with a few ferrites with the view to checking their actual characteristics with a Vector Impedance Analyzer.

CMC.JPG

The upper example is a string of ten #43 mix cores over LMR400 coax. It has a common mode impedance around 800-1200 ohms on the upper HF bands, dropping off to "it might still do something on 80m" and pretty well useless on 160m ... but depending on the antenna it is connected to, should handle power up to the limit of the coax (still to be tested). The next down has 2 turns through each of three #31 cores and a common mode impedance between 1000-1900 ohms between 6MHz and 41MHz, and should have a moderate power handling capacity (still to be tested) due to the three cores and only 2 turns on each core. The next choke (4 turns on a #43 core) has a common mode impedance between 1000-2100 ohms from 5.5MHz to 29.5MHz but just a low power handling capability due to the one core. The lower example is current balun wound from RG-58 coax in "W1JR style" and has a modest common mode impedance (about 700 ohms) but that is fairly "flat" from 1.5MHz to 40MHz (unknown core from the junk box). But it surely will not handle much power with the core being so small (1.5" dia x 0.25" thick). There are flaws in my measurement process, but the results give a general idea ... but there is no substitute for trying "something" in real life.

Keep in mind that the burgler alarm is not meant to be an HF receiver, so the real problem lies there ..... but getting the alarm owner to see it that way may be another story

73

The important bit is the alarm might be fitted and still not comply , stickers don't mean sqaut these days

[VK3MEG]

theo a quick question how were the audio reports on air?
whenI have had an rf isuue peopel could tell with the audio reports,

[VK2XSO]

I don't have anything to contribute to the solution, but can elaborate on how I caused a similar problem.
Many years ago while playing with a large tesla coil I'd built. The small coils I'd build leading up to a 1.2m coil were never earthed including that one.
They had a base plate which was a floating 0V reference for the circuit. With ~500kV flying around and occasionally striking things like the wall or the well isolated 240V power cable I decided
it was better to earth the base plate.
The effect was dramatic. Every house alarm, car alarm and door bell within 300m went off. I first got this idea at 11pm one night while testing the coil so it would not cause TVI.
The entire neighbourhood woke up and I never did really understand why so many devices over even a considerable distance were triggered.

Keen to hear what solutions turn up for this scenario in case I ever decide to build a another (bigger) coil.
the old tesla coil was give to my old high school many years ago.

[VK4TI]

I don't have anything to contribute to the solution, but can elaborate on how I caused a similar problem.
Many years ago while playing with a large tesla coil I'd built. The small coils I'd build leading up to a 1.2m coil were never earthed including that one.
They had a base plate which was a floating 0V reference for the circuit. With ~500kV flying around and occasionally striking things like the wall or the well isolated 240V power cable I decided
it was better to earth the base plate.
The effect was dramatic. Every house alarm, car alarm and door bell within 300m went off. I first got this idea at 11pm one night while testing the coil so it would not cause TVI.
The entire neighbourhood woke up and I never did really understand why so many devices over even a considerable distance were triggered.

Keen to hear what solutions turn up for this scenario in case I ever decide to build a another (bigger) coil.
the old tesla coil was give to my old high school many years ago.

A Tesla like that would be tantamount to a large spark gap transmitter ?

[VK2GOM]

I too built a large Tesla coil (back in the UK). It ran to about 1kV off the top toroid to air. It effectively transmitted on all bands at once. Listening on an FT817 it was just S9++ of noise; everywhere

I took it to the UK's 'GaussFest' and won the longest streamers of the event award

73 - Rob VK2GOM / GW0MOH

[VK5IR]

Well, I gave it a good shot today, but I wasn't able to set off any alarms.

Perhaps it was a coincidence, perhaps something else was setting it off, I dunno but I doubt it. I suppose only time will tell.