Re-radiation from antennas?

[VK4WDM]

I look after VK4KG, the ham station at the Townsville RAAF Museum (soon to become the Townsville RAAF Heritage Centre). Because it is a public site we have been very careful to have a wider than the calculated minimum EMR safety zone for our transmitting antennas - a multi-band HF vertical and a tri-band VHF vertical which are mounted on a shipping container 10m away from the back of the radio shack.

At the front of the shack we have two scanners connected to air band antennas mounted on short masts just above the roof line. These are strictly receive only.

I had a visit from one of the RAAF base's safety officers yesterday who was very impressed with our EMR compliance efforts including the VK3UM data sheets I gave him, but he did ask an interesting question: Do the receive only antennas also pose a safety risk due to "secondary radiation" from the transmitting antennas? (Our maximum power on HF is 100w and on VHF 25w).

The SI admits he does not know the answer and is very keen to hear what the RF experts in our community have to say. He is very "ham friendly" and is getting interested in getting a license so don't put him off with a silly cat fight and/or stupid comments.

73

Wayne VK4WDM

[VK1JA]

If antenna's pose a 're-radiation' threat then I think we're all doomed, as it wouldn't just be antenna's that re-radiate. Anything metal (light poles, letter boxes, gates, fences, power cords etc) would all be a problem !!

And if they did re-radiate they would have to do so whilst having 'gain' otherwise the non-radiating (direct RF) signal would be stronger anyway. Aaannnnddd, if they did re-radiate they probably would only do so at a particular (resonant?) frequency?

I'm just thought dumping and by no means an expert in the field, it'll be interesting to hear what the experts think.

[VK3ALB]

Do the receive only antennas also pose a safety risk due to "secondary radiation" from the transmitting antennas?

No

[VK4REX]

Interesting comment.

At Bald Hills near Brisbane where the ABC 50Kw MF station is, a long wire antenna at houses near the site will give you an RF burn. I have seen hams in the area use a resonant circuit matched to the long wire used to light a 40 watt light bulb.

Where I worked at Llandilo (a HF Ground to Air and International/Domestic RTTY station) if you were silly enough to touch the feeder from an unused rhombic you would get a good RF burn. Even opening the antenna switches on unused antennas would create massive arcs.

So I am not sure about definitive NO's but then power has to be taken into account we had at least 10 x 20Kw tx's lots of 5Kw and heaps of 1Kw tx's on site. At the time we thought it was a bit of a joke to send a new trainee down the north hall with a fluro bulb and watch him "sh.t" himself when the fluro ignited from the stray radiation. I am not so sure it so funny now.....

There was no such thing as site radiation folders etc in those days (70's)

Rex

[VK3ALB]

Hi Rex,

My definitive no was based on the situation outlined by Wayne. The situation is obviously different in the near field of a large radio transmitter.

[VK4REX]

Hi Lou

My comments weren't directed at at any post just a general comment.

But what makes this interesting is does RAAF base Townsville have a HF installation, where is it located and how close to the museum.

This may be another issue to consider and sort of what I was getting at.

I mentioned in a post an another thread that all the staff at the station I worked at only ever had female offspring so I think there is definitely something in the ether. I guess it depends on what you call an antenna

Rex

[VK3ALB]

Hi Rex,

Indeed. My Mother always told me too much of anything is no good. I believe family jewels are placed where they are for precise temperature regulation of the cargo. I've heard of the high incidence of female offspring in families where the father is exposed to high RF fields. Perhaps RF heating has some part to play in this variation? No doubt there are papers that attempt to explain the phenomenon.

[VK4GJW]

Having worked around RF fields for over 20+ years, communication systems, radar systems and others, I had 5 boys and one girl! Go figure!

[VK3KCX]

Interesting observations, I did my apprenticeship ~ 30 odd years ago, in a high RF environment... and back them all of the radio techs also all had female offspring.

The re radiation is not an issue most the collected energy would be dissipated in the connected RX front end.

- David

[VK4WDM]

But what makes this interesting is does RAAF base Townsville have a HF installation, where is it located and how close to the museum

Nothing with any power, all those transmitters are at remote sites. Plenty of lower powered stuff on all sorts of frequencies so we get a lot of burps and grunts at times and the noise floor can be very high.

I really don't think there will be any "public safety" from the scanner antennas with the sort of power we run and we only transmit intermittently anyway, but it was good to see him thinking about things that might be outside the usual square.

73

Wayne VK4WDM

[VK3BSF]

Using the power density formula and plugging in power input and gain of the tx antenna and then the distance away from the centre of the tx antenna will give u an idea of the power density of a reradiating rx antenna.

[VK5ZLR]

Using the power density formula and plugging in power input and gain of the tx antenna and then the distance away from the centre of the tx antenna will give u an idea of the power density of a reradiating rx antenna.

Yes I thnk so too, but the re-radiating Rx antenna would need to be resonant on the freq of interest, otherwise it will re-radiate virtually nothing.

[VK3ZAZ]

Calculations notwithstanding which have been found to be flawed how about get an emr meter and measure it.'

[VK3BSF]

small steps..using the range equation and for simplicity where all the incident power is returned to a bystander and gain of rx antenna is =1.., ..divide the incident power at the rx antenna by the distance to the bystander squared..ie Range(to bystander)^2...to get power density to the bystander

[VK2HC]

Hi Gents,
Many years ago I made a copy of the Arrow Antenna, Model # OSJ 146/440- 2 meter + 70 cm J pole antenna, no ground plane , omni directional with one coax feed line.
Still in use today we some times forget the basic of Amateur Radio.
Antenna_OSJ146-440_Manual@mkgjerhgerg5t.zip
Cheers Peter.

[VK3HJ]

The greatest risk, it seems is of Public Paranoia. Even with facts, it is hard to dispel that.

[VK3BSF]

Using the site below (v1.2 (2015-08-18) by Paul Evans, VP9KF) for calculations (where 28Mhz is the worst case) with the average power of a 100w ssb transmission at 28 mHz is say 30W, 2.1 dBi antenna gain (the multiband) and say 40 ft to the scanner antennae gives Estimated RF Power Density of 0.0067 mW/cm2..RF power density at the scanner antenna.

As the Maximum Permissible Exposure (MPE) is

a....1.153 mW/cm2 for controlled environ; and

b....0.2346 mW/cm2 for uncontrolled environ

so, if all the RF power Density (RFPD)is reradiated to a bystander within, say, 5m (500cm) of the scanner antenna the RFPD is: 0.0067/500^2 mW/cm^2 which is =0.0000000268 mW/cm^2...well within the limits of a or b above. THe results for 147 Mhz and 438 Mhz are very similar to the 28 Mhz one.. For 1.8 Mhz the results also look similar. Results will be less than above if only a fraction is reradiated.
Hope this helps and any comments on the above approach wud be more than welcome!!
Peter

http://hintlink.com/power_density.php
by v1.2 (2015-08-18) by Paul Evans, VP9KF

[VK2EFM]

Hi,
If you feel the issue is too complex to predict, the best approach might be to borrow or hire some E and H field probes and measure the RADHAZ levels. These can be measured at lower power levels (says 10W and scaled up).
If you share the base with defence they might be able to help with loan of equipment.
Regards,
Allan

[VK4WDM]

As an interim measure we have taken the good old hazard reduction strategy of "engineering down the risk"and moved the scanner antennas on to the container at the back of shack. Much longer feed line but still works OK. (even with RG58 which is all we had on hand that was long enough).

Having done that, I think the idea of "re-radiation" not only from passive antennas cannot be totally dismissed so we will carry out the suggested calculations and I will see what I can organize with the base communication guys about borrowing a measuring instrument.

I seem to remember Steve VK4ZAZ posting something in another thread about his bed springs? or kitchen door frame? radiating whilst he was transmitting so it might not be confined to antennas. I wonder about a metal toilet seat?

Interesting stuff!

73

Wayne VK4WDM

[VK2JDS]

An hours drive south of me is the 2bs AM broadcast transmitter site at eglinton (bathurst). Compared to an amateur setup its a massive amount of power.
Their audio has been heard coming from fridge doors, telephones and other kitchen appliances in the suburb around the transmitter site.
Its not reradiated , its just been rectified and become audible.
I wouldnt be worried about absorbtion by surrounding metal.
73 Dave