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Old 24th Jul 2023, 1:50 am   #1
Tyso_Bl
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Default Homebrew HF aerials, EMF compliance

Hello all,

I wasn't going to bother returning to Amateur Radio and have been away from things vintage for a while, I've finished repairing a rather badly abused Trio TS-830 transceiver (this is classic although not quite vintage!), works on a dummy load, and receives well as far as I can tell.

Since I was last active we now have to do the EMF assesment thing to keep things legal,
looking at the calculator app on the RSGB website I can fill in all the details except fot the aerial type from the dropdown box.

https://rsgb.services/public/software/emccalculator/

I've managed to rig up a random wire exiting the upstairs wireless telegraphy office through a window, fed by a matching arrangment at the window frame. The wire goes over the roof to the back garden attached to a support in the middle of the garden.

The question is what am I supposed to put in the box for aerial type?

I'm really at a loss with this one, the standard options there don't seem to describe what I've got to use.

I've had a good search online and just gone round in circles back to the calculator.

I can't believe I'm the only one who has encountered this situation, but haven't been able to find anything posted anywhere online about how to proceed to ensure compliance.

All help and advice welcome,

T
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Old 24th Jul 2023, 5:09 am   #2
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Default Re: Homebrew HF aerials, EMF compliance

Random wire operated end driven, no counterpoise etc. ?

There are two lengths of long wire in that calculator 40m and 80m. The clue that these are not bands but lengths is that you can set the band of operation seperately... they don't make this clear at all.

Pick the nearest and play with some numbers. Presumably the ends are nearest to your property boundaries. Note that the height of the ends sets the minimum horizontal clearance to human access.

My dipole goes to a tree in the front garden (planted for the purpose!) at well over 4m height and with 100W SSB or CW on 60m needs no minimum clearance.

Thee things in all honesty are very approximate, any attempts at actually measuring field strengths are remarkably inaccurate, and there are loads of assumptions in operating duty factors and mean powers.

Unfortunately, when cast into legal requirements they assume the literal application like things handed down carved into tablets of stone.

Transmitting into a random length long wire involves driving it against earth. Your earthing needs to be good or you get RF loose in the house. Impedance at some frequencies will be beyond the tuning range of the set's Pi-tank so you may be getting into ATU building territory.

David
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Old 24th Jul 2023, 6:51 pm   #3
G6Tanuki
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Default Re: Homebrew HF aerials, EMF compliance

I prefer the OFCOM calculator; it lets you experiment a lot more with the input parameters.

An end-fed longwire antenna, matched against a lossy ground, I would 'guesstimate' it has zero 'gain' on the lower and middle HF bands.

Equally, when I did the calculations for my end-fed-half-wave vertical on 28MHz, I 'reasoned' that the point of maximum radiation was the current-maximum point 1/4-wave up from the feed-point, so gaining me an extra 2.5-metres of 'elevation' above the risk-zone.

For me the big things are to "show your working" and "show your reasoning". My documentation includes things like the way there is a thorny hedge nearby which will help exclude people from any risk-zone, and notes that SSB audio modulation has something around 33% average power so my running 100W PEP can be cut down to 33W in the calculations.

Spreadsheets are fine, but the associated narrative is just as important!
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Old 24th Jul 2023, 7:47 pm   #4
Cruisin Marine
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Default Re: Homebrew HF aerials, EMF compliance

These things are all well and good, but if you are an eternally driven Antenna experimenter, how on earth can you even begin to comply with all sorts of reg's when using an exhaustless type of antennae in weird configurations being used then torn down (and sometimes put back up).
After all, isn't Amateur radio all about experimentation?
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Old 26th Jul 2023, 1:01 am   #5
Tyso_Bl
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Default Re: Homebrew HF aerials, EMF compliance

Thank you everyone for the replies, in particular Radio Wrangler (David), I'd missed that the 40 & 80M options were lenths rather than bands, I've run out 25M of wire which is all I have space for here, so used the 40M option. I've now got the ends over 7M up, so can manage without an exclusion zone at the powers I will use.

Regarding the end fed, I've a matching unit at the window frame where it exits the building, and a counterpoise along the gutter and drainpipe, the coax from the matching unit is choked off at both ends, interestingly (to me), I wound the chokes while the radio was on, and the drop in noise compared to signal was very noticable. Oh and yes there is a little RF loose inside the workshop not too serious, but some proper housekeeping should reduce it to as near to nothing as makes no difference. Having an upstairs wireless office is not ideal, but the coming sunspot cycle is probably the last I'll have an interest in, so anything is better than nothing!

Having completed the paperwork, I've done some tests and can tune up on the bands I'm interested in for now, reception is ok except on 80M where my active receive loop out performs the short longwire easily. I probably will arrange switching to use the wire for TX and the loop for RX, I did have a remote tuned homebrew "magloop" which covered 6.5 to 19Mhz (self resonant at that end), had it installed on the roof, guyed properly, but the winds here eventually battered it snapped one of the guys, and down it came, that was a cold nigh, about -6C and the embrittled plastics insulators shattered like glass.

Early days yet, but experimenting with aerials something that does interest me as its the main variable in the station.

Thank you for your quick and informative reply,

T
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Last edited by Tyso_Bl; 26th Jul 2023 at 1:08 am. Reason: Typos, and additional info...
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