You'll find that you're more likely to cause interference at higher power, so they're more likely to take interest if that happens.

Remember - your 7 Watts in a loft would be about ½W in free space!

Hi Albert, thanks for your reply.

Just on another thing, I've tested how far my new aerial covers and it seems to only cover half the distance my old aerial covered :-\ my old one was reflecting more power so I would have thought the new one would get more coverage as it was reflecting a lot less and seemed to therefore have a better match efficiency.

Do you know why this might be?

The old aerial had shorter element lengths as I based the lengths on the earlier online length calculator I was previously using. The new aerial I based the lengths on a different calculator which stated a longer length, which when I tested the reflected and forward power I had to increase further to get the lowest reflected power level. So I can only deduced that the length of the new one being longer has something to possibly do with it?

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Very nice video (if you don't mind this American guy way of speech) about SWR and antennas:
https://www.youtube.com/watch?v=oejsHzurzv4

Hi pjeva,

Thanks for your post, I will have a look at it as soon as I have a bit of spare time.

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your aerial is too long buddy! at 81cms for each element it would problaly be closer to resonating on below 87mhz than your chosen frequency. What is the dipole made out of? the guage of the wire thickness

Hi Radionortheast,

I made my aerial as per the instructions here:

http://www.doityourselfchristmas.com/wi ... ntenna.pdf
But did the length as per my previous post.

I did fold the ends of the wire back on them selves to the shorter length and then twisted
the excess wire back around and then tested the reflected power but it was reflecting a lot.
That's why I then extended the wire back out to the ~81cms as that was giving me the smallest
reflected value.

Should I have really got the length back down closer to the calculated length and then increased/
decreased the wire length by small amounts to then try and find the smallest reflected power
value at the calculated length?

yes you should go back down to the calculated lenght
i’ve just used the calculator the lengh 73.3cms on 97mhz.

the reason I mention the thickness of wire, the thinner your elements are the more their going to get effected by surrouding objects specially inside a loft, talking about the width of the copper,(excluding insulation) I trust it is at least 2mms. You could double up on the amount of copper quite easly... the easyest thing for you to do would be to cut it back down to calculated figure, go up in the loft early on a morning late on a night when its cool, put transmitter on your chousen frequency look at the swr meter move the aerial, try to find a place where its the lowest.

in the instructions it says tv coax, I trust you are using 50 ohm coax, you problaly won’t beable to meaure 75 ohms, it may cause problems going over a few watts.

the outer ground sheath of the co-ax should go to the bottom element, a quick way that you can check your aerial is been effected by surrounding objects is by fliping it round so the ground is at the top see if you get the same reading.

(it maybe just the diagram the coax looks to about a foot away from the bottom of the aerial) coax should be at least the lenght of your dipole element away from the bottom,(problaly means 2 or 3 feet) actually the cable should be lead away flat horizontally but its not possible with a vertical dipole.

i’m curious to know you mentioned you’d been broadcasting for a year, i’m wondering what your old aerial was and was it in indoors, in your room, what results where in comparsion..you don’t have to share any of that stuff with me, its useful for you.
You should be writting down all of what your doing theres nothing more annoying getting a good result then later on not knowing what you did.

The way I would do it is this... Tune your antennal in free space (a friend's garden without nosey neighbours perhaps?) amd you'll probably find it's a lot closer to the calculated value. After doing that, put it in your loft amd don't worry about the SWR, it will be higher but at least the antenna will be at resonance.

One "anti-suss" aerial I used was a 1/4-wave vertical on a short pole, mounted to a chimney with a TV aerial lashing kit. The short pole just happened to also be a 1/4-wave long - look up "sleeve dipole". From the ground, it was a lightning rod, protecting the house! After a bit of tweaking, the match was excellent, and this aerial was used intermittently for several years without being noticed for what it was!

Remember - an indoor aerial will waste 90% of your power!

All:
Thanks for your replies and information, it is much appreciated!

Radionortheast:
I will have a go at re-cutting the elements back down to the correct length
and then do as you say, move it about and try to find a place where the SWR
is lowest.

Yes, I am using RG58 50 ohm coax cable and not TV coax.

Currently I have the coax leaving the aerial horizontally and it extends about 350mm
before it then drops down to floor level.

The old aerial was made in exactly the same way as the new with the exception
that I had cut the elements to the calculated length and instead of using a Pawsey stub coax balun
I twisted the coax around a ferrite rod just after where the coax joined the elements.
This aerial is still mounted in my loft and I tested it with the SWR meter last week
in situ and if I remember correctly I was getting Pfor = 0.9 watt and Pref = 0.1 watt.

I have been keeping notes on what I have been doing, so do have it as a reference for
the future.

Albert:
I may have a look in the future, when I next get a bit of free time, to try another style of
aerial and may be site it outside somehow. The pain is I don’t have a chimney on my house
so may have to look at alternative external mounting options.

Hi again,

I have now got a new TX and have set it at 7watt (can go up to 15watt)
along with a ground plain antenna loft mounted.

Does anyone know at what wattage RF tissue heating becomes a problem for people
in the near vicinity? i.e. people in the house if you are broadcasting from the loft?

I've had a look around the net but cant seem to find any exact information
that tells me what wattage is dangerous. Just want to make sure i'm not
going to cause problems to the people in my house.

highly unlikely to cause any harm unless you touch it or cms away unless anyone else wants to chime in , remember theres a null under the aerial, means less signal, would of thought your problem would be interference, some audio amplifiers aren’t good theres always a chance of it been heard in your nieghtbours audio equipment, been heard in their speakers, with the high field strenght. I used to live 500 meters away from a mast it broadcast all things don’t know the power but you would often hear the police talking on my sisters hifi when it was idle.

(so don't worry you won't cook anyone! would of thought the only way to cook someone would be with microwaves maybe some infrosonic sound it can heat up molecules )

The net won't tell you what wattage is dangerous because it is a contentious issue. Tissue damage from R.F. increases with frequency, field strength, duration of exposure and a bunch of other factors that are less easy to define and have been worked over by conspiracy theorists - they wanted to prove that mobile phones and wi-fi signals can cook your brain. At these frequencies the body will have trouble absorbing the radiation because the wavelength is large by comparison - try walking around a TV aerial and see what happens the reception then do the same on FM. This whole area is muddied by wanker vegan wholefood sandal wearing American soccer moms - there is no point looking at anything on the net about this, it is corrupted. I can show you on bbceng.info photographs of BBC engineers enjoying retirement after a lifetime of working 40 hour weeks with 250,000 Watt TV and FM transmitters right beside them keeping their tea warm. In short - if a bloke worse than Albert is still going then we all safe enough... Sorry Albert!

We used to joke that "microwaves frizz your heirs"!

Cheers for the info ppl.

I do get a load buzz from my M Studio monitors which are connected to my PC when
I first turn them on, but it stops after about 2 seconds. TV which is FreeSat is not affected
nor my FM tuner downstairs. So hopefully the neighbors are OK too.

mine over loads all stations till radio 4 on 93.5 but over the road radio 2 on 89.1 starts working on my portable though was putting out 25 watts DAB signal seems unaffected and even london 1 2 and 3 were coming in on the usual 8 bars.
I use 87.7

I've got a 1/2 NRG vertical that needs rebuilding.

Accidentally forgot to turn the power down after a test and melted the plastic thingy.



The signal was booming!


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