Loft mounted halfwave pipole RF saftey

[halfwave]

Hi all,

Just wondering how safe it is to have a half wave dipole in my loft being fed from a small 1watt FM transmitter. i.e. would people in the the bedrooms directly beneath the dipole be adversely affected by RF energy.

Or is 1 watt power so low it will not matter even with gain of 2.2 from the aerial? Just want to make sure I'm not going to fry people while they sleep [WINKING FACE]

Also if I pushed the power up to 7 watts from the transmitter how much of a difference would that make?

Any advise would be appreciated.

|-| /-\ 7 |= \/\/ /-\ \/ 3

Sent from my Nexus 5 using Tapatalk

[s2000]

You wont fry anyone with that low power but you will probably have a poor swr reading with it mounted in the loft, it should be outside where it can radiate properly. Also a standard single dipole has no gain as far as I am aware, you will only get gain by stacking them or using a different antenna.

[McDonalds]

My SWR is around 1.3 to 1.5 in my loft on my aerial have gone right up to 25 watts most but the music starts to play out my speakers even when the amp is turned down LOL.
The signal was getting out a good 10 miles which is not bad its worse in the garden because being built up so I get out more in my loft.
on 5 watts there is no buzzing at all.

[halfwave]

You wont fry anyone with that low power but you will probably have a poor swr reading with it mounted in the loft, it should be outside where it can radiate properly. Also a standard single dipole has no gain as far as I am aware, you will only get gain by stacking them or using a different antenna.

Hi s2000,

Thanks for your reply.

The problem I have is that as I'm transmitting from the house only locally, and don't want to slap the dipole on my roof as don't want to draw attention.

I have limited options available:

Keep it mounted in the attic.

Push more power say go to 7 watts.

Or mount the aerial in my garden but keep it out of sight from prying eyes. Which would mean it being not as high as where it currently is in the attic.

So if I went up to 7watt and kept it in the attic would that get me out better?

Or would it be better if I mounted the aerial in my garden say about a Meter and a half from the ground. Away from other objects. Than having it in the attic.

Or if I mounted it half way up the house on the rear wall, may be a possibility, but I guess I would have to mount it horizontally so the signal is directed away from the wall.

I know the general rule is get the aerial up as high as possible and away from other objects. But unfortunately I don't have this luxury.

Sent from my Nexus 5 using Tapatalk

[SundayGroover]

With a TX antenna in the loft you should only expect a few hundred metres of semi decent signal from 1 watt. 7 watts would likely show little improvement. Shame as that's my only option these days (since moving) if I decided to resume transmissions!

[s2000]

My SWR is around 1.3 to 1.5 in my loft on my aerial have gone right up to 25 watts most but the music starts to play out my speakers even when the amp is turned down LOL.
The signal was getting out a good 10 miles which is not bad its worse in the garden because being built up so I get out more in my loft.
on 5 watts there is no buzzing at all.

Sounds like you have saturated your house with rf, you can get all kinds of strange things happening especially at higher powers lol

[teckniqs]

It will obviously work much better above the roof but without that option I'd definitely say go for the loft option than the small garden pole. 7w isn't enough power to worry about.

....I've messed around with dipoles in lofts in my late school years and reached some surprising distances with very low power, it can be done!
I used to live low down near a river in Byfleet, Surrey and had a little dipole in my loft for the occasional test etc.
With 1w at the top end of the band I was able to cover the whole area clear as far as the other side of West Byfleet which is 2 miles away, and with just 4w I was able to cover 90% of Woking town which was a good 5 miles away.
The SWR was perfect too after placing the antenna carefully in the right spot, I've even made some simple dipoles out of two pieces of centre fed wire with coax for use in the loft with good results.

With any more than one watt you could hear the audio down the phone and my Dad would come straight upstairs and make me switch it off.

[s2000]

You wont fry anyone with that low power but you will probably have a poor swr reading with it mounted in the loft, it should be outside where it can radiate properly. Also a standard single dipole has no gain as far as I am aware, you will only get gain by stacking them or using a different antenna.

Hi s2000,

Thanks for your reply.

The problem I have is that as I'm transmitting from the house only locally, and don't want to slap the dipole on my roof as don't want to draw attention.

I have limited options available:

Keep it mounted in the attic.

Push more power say go to 7 watts.

Or mount the aerial in my garden but keep it out of sight from prying eyes. Which would mean it being not as high as where it currently is in the attic.

So if I went up to 7watt and kept it in the attic would that get me out better?

Or would it be better if I mounted the aerial in my garden say about a Meter and a half from the ground. Away from other objects. Than having it in the attic.

Or if I mounted it half way up the house on the rear wall, may be a possibility, but I guess I would have to mount it horizontally so the signal is directed away from the wall.

I know the general rule is get the aerial up as high as possible and away from other objects. But unfortunately I don't have this luxury.

Sent from my Nexus 5 using Tapatalk

Oh I see, well I would leave it in the loft but if you start to run 7 watts, might be an idea to keep an eye on the swr readings. Do you have a swr meter? It will tell you what power is being reflected back to your transmitter. You dont want too much reflection as the transmiiter will get a lot hotter than it would if running with good swr readings etc...

[halfwave]

Hi Teckniqs,

Thanks for your reply.

I live a low lying area so find it hard to get out very far. In one direction I get out about 3/4 - 1 mile
in the others only about 1/3rd of a mile, due to ground level rising up quite quickly from
where I live.

At the moment I have the dipole mounted vertically, but I guess I could mount my dipole horizontal
and point it in the direction that can currently get out furthest to and see if
that pushes the signal out further. Or with 1 watt would that not really make much of a
difference?

The main reason why I have not gone horizontal is because of it being in my loft
and that a bedroom is directly below so the maximum radiating pattern would be on a vertical
plane, rather than at its weakest (on a horizontal plane), when the aerial is mounted vertically.

Just a though, has anyone ever mounted a dipole diagonally? and would that still kind of give
me omni directional as well as a bit of directional if pointed towards the area I would like to cover?
Or should you just go vertical or horizontal.



s2000:

I do not have a SWR meter. But I have put a simple balun on the arm of my dipole, by winding
the coax cable 4 turns around a ferrite rod, as well as creating three loops on the coax cable
held together tightly with cable ties, about three feet before the dipole arm, to hopefully
prevent too much reflection.

Is an SWR meter expensive? and is there one that does not cost too much
that you could recommend. Also could you explain how do you use one correctly.
I guess you would hook it up in-line between the transmitter and the aerial?

FYI.
I am still quite new to all this, so any guidance would be appreciated.

[bristolpirates]

I guess I could mount my dipole horizontal
and point it in the direction that can currently get out furthest to and see if
that pushes the signal out further.

You'll still be pushing the power out in an omni-directional pattern, but partially into the air and down into the ground. You will need to add a reflector and turn the dipole into a yagi.

[radionortheast]

with 1w you could use a halo outside, used to have one that was a perfect match on 87.9, i’ve got halos since and they where tuned to different frequency so it could a stab in the dark. (if theres no means to test it before it gose up you’ll be stuck with an aerial which might be a dud will limited to 1w) I could get the signal strong 4 miles away with it, not much of the energy went into the house, I could tune to weak stations dx with my tuner.

The raidation pattern was funny maybe only got a coverage 100 meters throught buildings, it would only cover one area of town, it would come up stronger in the surrounding countryside. I would often have hold my radio horizontally to get the stronger signal, car radios didn’t like it...

when got the nrg 4w, it blew the transistor when connected up to it, i would use an aerial in the loft it was kind like a dipole, the main element was cut for frequency, then had a longer element pointing in one direction.. it covered the town afew other towns near by which was kind of cool. Can’t say I had much problems with rf leaking into the house.

when i moved, i started back up using loft aerial, the loft wasn’t full height, would usually run the pcs 15w, but would take the fan out and use it at a lower power level afew watts, any heighter and it would knock out tv. (when tv aerials are close to a transmitting aerial they overload) Before it I would use the nrg 4w, I was suprized how far it would go, the hill was half the height nolonger got coverage in any of the towns, except for a small section in one, the signal would be there but below coverage levels. Even so it would go out a fair way south and east, managed to have an audioable signal 15 - 20 kms away east and south east it went over 20 kms. Even thought the loft may take out afew db, the aerial will still have line of sight will travel..if the loft is high up there won’t be any problems with coverage.

apparently if you make an aerial which has radials on it the bottom, you won’t get much raidiation below it, i’ve heard albert h talk about it constantly. mightbe worth giving it a try, if all else fails or if you have plenty of aerials, might just be worth bending the elements round into a halo shape of your dipole sticking it on your chimmney!

[Albert H]

There are lots of simple designs for subtle aerials - you don't need to have an obvious dipole on your roof!

One trick we used to use was to mount a TV aerial on top of a long mast. ¾-wave down from the top we'd fit a ¼-wave stub, clamped off the side of the mast, mounted vertically about 5cm away from the mast. The coax centre would connect to the mast, and the outer to the stub. The Band IV Yagi TV aerial at the top of the mast didn't affect the real aerial at all. The mast was the radiator - or at least the top ½-wavelength of it was! This was a pretty effective J-pole - better than a dipole (by about 2 dB), and not obvious from the ground. We even used the Band IV Yagi for the link receive aerial on some installations! I ran the transmit coax down the inside of the mast, and included a 9-turn coax coil inside the mast, just below the feedpoint.

You can buy the masts up to about 10m long, and a TV Yagi is cheap (<£5 from CPC). The masts often come with clamps and Rawlbolts for attaching to a wall or chimney.

You will lose about 80% of your signal with an aerial in a loft. When it rains, and the roof tiles are wet, this will rise to over 90%. It's a huge waste of expensively generated RF! Build a concealed vertical (like the one described above) or a "Flowerpot" aerial, put it outside the building - as high up as possible - and you'll be amazed at the improvement in coverage.

In my youth, I used to use a loft-mounted 3-element yagi to get my signal into the nearest town (about 3km away). To get a reasonable signal on an average portable, I had to run 12 Watts. When I put a J-pole on the roof, I got much more coverage - three towns and a couple of villages - in stereo with only 2 Watts!

You really do need an SWR bridge. You can get one for 2m amateur radio for around £10 - £15 and this will work perfectly well at Band II.

[s2000]

s2000:

I do not have a SWR meter. But I have put a simple balun on the arm of my dipole, by winding
the coax cable 4 turns around a ferrite rod, as well as creating three loops on the coax cable
held together tightly with cable ties, about three feet before the dipole arm, to hopefully
prevent too much reflection.

Is an SWR meter expensive? and is there one that does not cost too much
that you could recommend. Also could you explain how do you use one correctly.
I guess you would hook it up in-line between the transmitter and the aerial?

FYI.
I am still quite new to all this, so any guidance would be appreciated.

Yes, I am not an expert but I believe a balun is a device that is used to prevent the feeder (ie the coax) becoming part of the aerial system. If you don't use one, it's possible to have a very good swr reading (e.g low 1.1:1 or 1.2:1 etc) but its also possible that the coax may be radiating part of the signal (along with the antenna). This will effect the radiation pattern and may cause other unwanted problems. Ideally you want the antenna to radiate all the signal and the coax to pass the rf from A to B without radiating any of it. A typical balun for a dipole is a pawsey stub balun. This allows you to connect an unbalanced line (such as a coax) to a balanced line (dipole). Basically a coax is described as 'unbalanced' as the two conductors are not identical e.g the centre conductor is a different geometry to the outer screen (sheild)... The dipole is slightly different as both conductors are usually identical (ie same diameter, length etc) so these are 'balanced'.. the balun will help match these together and prevent the transmission line from becoming part of the antenna..

I would definetely buy a swr meter, it was one of the first pieces of equipment I bought when I first started building bits. It can be used for tuning up rf amps (in conjunction with a 50 ohm dummy load) and also for tuning antennas for best efficiency. It connects in between the transmitter and antenna and will tell you what power is coming out of the transmitter in watts (forward power) and what is coming back from your antenna (reflected power). The swr is a ratio of these two measurements. Ideally you are looking for the lowest swr reading which equates to the best efficiency. If the swr is too high, it means that a portion of the rf energy your transmitter is generating is being reflected back, the transmitter will then have to try and lose this as heat so will be running hotter than it should be..

[halfwave]

Thanks again for all your replies.

I will have a look and get myself a SWR meter
and see what my current dipole's value is.

My transmitter and aerial connectors are TNC
type will this make a difference in which meter
I should get?

Also do I need to find one that has a range that covers the frequency
I am transmitting on?

I did a search in Google and ended up at ebay
but there are so many I am not exactly sure which
one would be suitable.

If possible could someone point me in the right direction
of which is the correct one for me to get, either on ebay
or another website.


Hopefully when I get a bit of free time I will have a go at
building another aerial as per Albert's recommendation.

[s2000]

Yeah, you need to get one for the frequency of interest (vhf), dont buy anything marked 'cb' swr meter, these are for lower bands (30mhz ham bands) and will give you unusual readings if used with your transmitter.

The one I have is the diamond sx200, it is still around today but is badged under different names like 'maas' or 'avair'...

http://www.ebay.co.uk/itm/AVAIR-AV-201- ... SwrklU7zg9

Most swr meters will have SO239 sockets on them so you would need to either buy a few TNC adaptors or make up a few coax leads, one with a TNC plug on one end and a PL259 Plug on the other (allowing you to connect transmitter to the swr meter) etc...

There is a good video that breifly explains how to take a swr reading...

https://www.youtube.com/watch?v=jRWKZskxcs0

Please note this is a 'ham' operator, when he says "key the radio" he is basically pressing the button on the mic which turns the transmitter on.

[halfwave]

Hi s2000,

Thanks for the info.

I found this one on ebay:
Maas RX 20 3.5-150MHz SWR Meter HF & VHF 2 meter Avair AV20
http://www.ebay.co.uk/itm/Maas-RX-20-3- ... Swcu5UNpdw

It's the two needle version. and also found the following which explains how to read it.
http://www.hamuniverse.com/readcrossneedleswrmeter.html

Would the two needle one ive shown above be ok? and is it easier to read a two needle
than a single needle?

[Albert H]

I use a two-needle SWR bridge - it's actually very quick and easy to read, and you can see the reflected power fall as you adjust the aerial.

Useful trick: get a small (1 Watt or so) transmitter on your frequency of choice. This will allow you to make aerial adjustments at low power before you connect your higher powered (and more expensive) transmitter to the aerial system. If the aerial is really badly mismatched, it is quite possible to do damage to the business end of a higher powered transmitter. Many of the Dutch transmitters actually have SWR protection built in, so that if the transmitter is connected to a really bad aerial (or no aerial at all!), it won't fire full power into a bad load.

My little test rig runs off a couple of Nokia mobile phone batteries, delivers about 1 Watt, uses a 5 Watt output device (very under-run) and is a VFO - tunes using a multi-turn pot. It's stable enough for testing - to within about 5kHz - but not for broadcasting. It's in a small diecast box with an SWR bridge built-in. The output stage runs cold, and doesn't care about bad matches, so it's great for building and adjusting aerials!

The other bit of homebrew test gear is a field strength meter, with an audio output and a telescopic aerial. It beeps, and the frequency of the beeps and their repetition rate increases as the field strength goes up. When I'm testing, I put this thing a few metres away from the aerial I'm testing, and the changes in field strength are obvious as I adjust.

[MiXiN]

Hi s2000,

Thanks for the info.

I found this one on ebay:
Maas RX 20 3.5-150MHz SWR Meter HF & VHF 2 meter Avair AV20
http://www.ebay.co.uk/itm/Maas-RX-20-3- ... Swcu5UNpdw

It's the two needle version. and also found the following which explains how to read it.
http://www.hamuniverse.com/readcrossneedleswrmeter.html

Would the two needle one ive shown above be ok? and is it easier to read a two needle
than a single needle?

That cross needle meter is OK, I have one, but the seller isn't. There are many complaints about him on the Transmission 1 forums.

Can't go wrong with cross needle meters like the Avair, as they show the power and SWR simultaneously without having to flick any switches or alter any knobs.

Go for that Avair as it's cheap, accurate enough, and has served me well for many years in addition to my more expensive Daiwa one.

[s2000]

Yeah the two needle ones are good too, I haven't played with one yet but seems good you can monitor both forward and reverse at the same time which is handy..

My only concern is on the spec of the device it says "Sensitivity 3W for full scale deflection".. I am going to presume that means that if you use lower power than 3W you may find the meter is inacurate. You say you have a 1W rig but can power it up to 7W, you would probably find that you would need to run it at 7W to get a true swr readings.

On my single needle meter it is similar, you need to have a certain amount of power in order to get the meter to full scale deflect when doing the calibration. I think my meter may be more sensitive as when I was younger I built a 300mW exciter and set it up using the swr meter and can't remember having any probs getting the meter to full scale deflect on 'cal'.

It would have been nice if the meter had an extra low setting, currently it has 0-30W and 0-300W. A 0-3W would have been a handy addition. What you will find is when you run power into the swr meter and you are on the lowest scale (0-30W) the needles will not move much (especially if running 1w).

I guess it depends on what power you plan to run the rig at eventually?

[halfwave]

Once again thanks to you both for the your advise, it's much appreciated.

Albert H:
Thanks for the tips and pointers

s2000:
At the moment I only have a 1 watt transmitter, but at somepoint in the future I
may go up to a 7 watt one, dependant upon a few factors. So I will have to test SWR
with 1 watt. Hopefully that will be enough to see if its really out in a big way or not.