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Shield required for FM transmitter

coo1guy

New Member
I'm planning to set up a 15W FM transmitter for a community radio with an omnidirectional antenna.
The range of this kind of transmitter is a couple of miles without obstacles.
As FM radiation apparently increases the risk of cancer, I'm wondering what kind of shield I would need to protect the staff near the antenna.
Which metal and how thick?

Thanks
 

alec_t

Well-Known Member
Most Helpful Member
Welcome to ETO!
1) Do you have a licence for the transmitter? Forum rules ban assistance with illegal activities.
2) Is that 15W the transmitter input power, the ERP or the actual radiated power? According to the FCC, "an ERP of 100 watts corresponds to an actual radiated power of 5-10 watts, depending on the type of antenna used." It is the actual radiated power, and the strength of any antenna side-lobes (bearing in mind that a nominally omnidirectional antenna may not be truly omnidirectional), which would determine the type and degree of shielding needed.
3) What is the transmitter frequency?
4) What height mast will the antenna be mounted on?
5) What form will the metal shielding take? Presumably something more than tin-foil hats?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
As FM radiation apparently increases the risk of cancer, I'm wondering what kind of shield I would need to protect the staff near the antenna.

As there's never been any study which has ever suggested FM radio causes cancer, it's a pretty pointless question. If any such unlikely event did occur, it's a LOT, LOT worse holding a mobile phone right next to your brain.

However, assuming this is legal and licenced, then the aerial should be outside and fairly high - so no where near any people anyway.
 

coo1guy

New Member
As there's never been any study which has ever suggested FM radio causes cancer, it's a pretty pointless question. If any such unlikely event did occur, it's a LOT, LOT worse holding a mobile phone right next to your brain.
 
Last edited:

Nigel Goodwin

Super Moderator
Most Helpful Member
One study out of the entire planet? -with hundreds (if not thousands) of studies having found otherwise. And pretty vague results and conclusions from this study as well :D
 

ronsimpson

Well-Known Member
Most Helpful Member
Life causes cancer.

The distance of your station is very much related to the height of the antenna. So put the antenna up high, not on your desk. ;)

Put the antenna in the roof, or higher. There will be almost no radiation going down from the antenna. It is designed to radiate out not down. As Nigel said, 2 to 3 watts from your phone pressed against your ear is much worse. A WIFI router 1 meter away on you desk is 10000x worse.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
As FM radiation apparently increases the risk of cancer, I'm wondering what kind of shield I would need to protect the staff near the antenna.
Any place that calls a RF from a frequency modulated transmitter "FM Radiation" is pretty guaranteed to be a conspiracy or scam advertising site.

I canot imagine any legitimate site using that phrase, and there is no legitimate evidence that low power RF exposure (eg. within legal limits) is in any way harmful.

Ionising radiation is harmful; that's UV light and frequencies above, such as X-Ray and gamma ray.
Any increase in cancer since the 60s could caused by anything from pollution to residue from nuclear tests.
 

Tony Stewart

Well-Known Member
Most Helpful Member
15W VHF will cause Red eyes in a day if used a lot at arm's length. I know a tech who got that from just the leakage of tuning a 100W Amp with no lid. The Design engineer, then put in tuning holes in the lid. Circa 1975. Eventually that causes cataracts.
 

Tony Stewart

Well-Known Member
Most Helpful Member
By stacking dipoles properly spaced, the torroidal shape of energy radiated squashes and amplifies like a flatter donut to spread out to horizon rather than up in sky and down in ground.
It needs to be above the tallest tree. But doesn't need to be this big.
1631717927555.png
 

augustinetez

Active Member
There are plenty of safety calculators around around that will tell you how high the antenna must be to maintain a safe distance from.

15W in to an omnidirectional antenna (without knowing what exactly the antenna is) will give a distance of approx 5 metres worst case, so mounting above the roof will put you around the safe zone - higher is better.
 

ronsimpson

Well-Known Member
Most Helpful Member
A lot of what is being said depends on the antenna. Please post a picture of the antenna.

It is interesting "15 watts" because that is a strange number. I have not worked FM stations in years but there is a "10W" class and 15W is not in that class. This makes me question everything. It is not clear if the transmitter is 15W or the ERP is 15W.

Here is the pattern for a antenna I had and you can see there is almost no energy going down.
1631763269781.png
 

unclejed613

Well-Known Member
Most Helpful Member
And pretty vague results and conclusions from this study as well :D
areas with many FM stations are mostly urban areas with a LOT of other factors that could account for increased melanoma rates... there's a saying "confluence doesn't equal correlation"... a lot of these "studies" are done on government grants with no peer review...
 

audioguru

Well-Known Member
Most Helpful Member
That is it!! I got cataracts about the same time I designed my 282mW FM transmitter. I used it for no more than 1 hour and it was near me for less than half that time.
Millions of other people my age got cataracts and had nothing to do with an FM transmitter. Then it was the person's age that caused cataracts, not an FM transmitter. Cataracts is not cancer.

Many people got melanoma cancer and never had anything do do with an FM transmitter.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
15W in to an omnidirectional antenna (without knowing what exactly the antenna is) will give a distance of approx 5 metres worst case,
Huh???

Inverse square law:
15W at 1m results in the same field strength as ~40mW at 5cm.

A normal mobile phone typically runs ~500mW at roughy 5cm or less from your head.

That's equivalent to 200W at 1m
(20 times further away, so 20x20 = 400 x power ratio.

A phone in your pocket running WiFi at 50mW, say 1cm from your skin? at 1m, that field strength would need 10,000x the power; 500W

Handeheld VHF radios, often

15W RF at just arms length is trivial compared to many other sources, even allowing for the worst case with a directional antenna.

Worry about sunlight and UV from that. No normal person will ever experience levels of EM radiation anywhere near what you get from sunlight, or as heat from a fire.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
A normal mobile phone typically runs ~500mW at roughy 5cm or less from your head.

I thought they ran about 2-3W maximum?, power is dependent on how strong the signal is at that exact moment, and varies as the signal strength does.

This is why the worst thing for a phone is to take it where there's no signal, because it turns the power up to maximum as it tries to connect - so drains the battery more.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
Yes, higher with a weak signal and lower with a strong signal; that's just an approximate mid-range value.
 

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