kite antennas - questions

Equipment
1

hi all - as plagued by noise in the city and based in the far SW + disabled - I don’t tend to do much radio at all these days - more building … any hoo - kite antennas - always wanted to try one so bought a sotabeams kite for the days when my good friend dave 2e0dtc and I go out on our mini expeditions - which are usually more test days of kit than anything else but none the less fun

so to kite antennas - specifically the very important static bleed resistor - I run QRP - so 6 w max from an FT818 - or a couple of w from a venerable ft290mk2 - now my question is just how big power handling wise resistor do I need ?? - digging through the junk box has proven fruitless for 1 M ohm + resistors of more than 1/2 w - will these be enough for qrp ?? - i could probably find bigger carbon ( non inductive ) ones on flea bay et al - but they tend to either want silly prices for a few - or one to buy 200 of them - usually from china which takes time to get here

oh and if you wondering about the 290 r ?? - yes you can also use "long wires " on 2 meters - or even better some sort of phased array - using a suitable atu - but that’s a different subject - and hoisting these antennas aloft with a kite - a different subject again - very much experimental stuff - which I love

thoughts please - thanks de G6URM brett - Plymouth

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Some stuff on here about a static arrestor…

2E0YYY

3

Oh! 2014 conditions - those were the days … :disappointed_relieved::weary::sob:

Great idea a kite antenna though, ideally for a two or more person activation though I would say, one operates while the other keeps an eye on the kite.

Looking at the picture in Mike;s article Brett, it looks like high wattage resistors are required - I suspect the transmission power is no so much a deciding factor as the amount of static electricity that builds up on the antenna wire.

73 Ed.

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4

Have you read the article on kite antennas on the Sotabeams web page?
Richard sells the resistor you require, search resistor to find it.

https://www.sotabeams.co.uk/kites-faq/
Mike

5

Ohm’s Law will answer your question.

6

yes - but others say different re the value required - i have 2 w ones - ( now found some in the old stock in a shop in plymouth - 50 p for a handfull assorted high values ) - there is a lot of differing info out there some even say use those high power cement ones which i have loads of from maplins close down sale - but i thought they where wire wound and therefore inductive ? - obviously i need a definitive - "been there done that - radio survived ok " answer as i don’t want to blow the 818’s front end - mind for first test methinks the old clansman 320 may get used - take 110 volts up the input !! - built to survive emp bursts .lol

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7

thanks but if it where that simple - i would not be asking

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8

I’m not sure it’s so trivial Andy. Whilst the resistor does not need to dissipate any significant RF power it does have to withstand a pulse from a potentially very high (kV or even 10s of kV) electrostatic charge [edit - when you connect it up I mean]. Just how big is the current pulse will depend on the inductances - wire wound no bad thing I’d say. Of course the pulse is extremely short, and perhaps continuous power rating has little to do with surviving this. I suppose I’d go with whatever I found in the junk box, but if I had a choice of two I’d use the bigger one - harder to lose in the mud anyway. :slight_smile:

9

It could not be simpler.

The antenna is assumed to be 50Ohm. The output from the 818 is 6W.

Ohm’s Law V = IR and P = V^2/R

6W = V^2 / 50

50*6 = V^2 = 300

Therefore V = 300^1/2 = 17.3 V

With a 1MOHm bleed resistor the current is

i = V / R so 17.3 / 1000000 = 17.3uA

and the power is IIR = 17.3 * 17.3 = 299uW. So the 1/2 watt will be fine.

These are the steady state conditions from your 818 driving the antenna.

The resistor needs to be non-inductive, so a carbon composition would be ideal. Should there be static induced on the antenna, the 1MOhm will allow the current to discharge to earth and will stop the voltage rising to a point it causes a problem to solid state devices in the RF stages. But you do need a real earth to discharge to, so ground stake is needed.

What it can’t protect against is any kind of close nearby lightning strike or a direct hit. In those cases all bets are off.

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thank you - and i will deffo not be kite flying if there is any chance of thunderstorms - been way too close to that already in life - had a work colligue killed by the stuff - so nope nice blue sky nice day kite flyer me . -

11

Very wise. Static risk is not confined to kite antennas! I’ll never forget one occasion when I set up /P on The Begwns (GW/MW-025) with my FT-817ND and a simple half wave dipole for 28 MHz on top of my 7m fishing pole. I had only been on the air for 5 minutes when I started hearing loud clicks in the receiver, initially at the rate of about one click per second, but rapidly increasing until it was a continuous “brrrrr” sound. Typical rain static noise, even though it was not actually raining overhead. I thought it prudent to unplug the antenna to protect the rig from damage … and (with hindsight) with enormous risk I unscrewed the PL259 from the back of the transceiver. To my alarm, I immediately saw bright sparks jumping across the inner and outer conductors of the PL259 plug, at the rate of at least one spark per second. I did consider running away, but finally decided the least dangerous option was to lower the fishing pole so that the antenna was lying on the ground, at which point the sparking stopped!

You would never have thought that such a short antenna (5m long) and only 7m above ground would pick up enough static to generate huge sparks! Once bitten … twice shy. I shall never again set up /P antennas if there is thunder activity anywhere near.

73,
Walt (G3NYY)

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12

Sparks by static charge on a Groundplane-antenna. - YouTube - seen this from wire antennas - and we had a massive thunderbolt land across the road demolishing a chimney made of solid granite blocks when i lived on dartmoor - my kit was ok as unplugged - but another ham 's radio got burned out - again unplugged - but the spark arked a good 12 inches - i saw the evidence when he returned from holiday - lucky i did not called out in my other job to put the house out which was a couple of streets away - so yes much respect for the stuff …

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Hi Walt,

I had a lot of fun with kite antennas. Lost a 1.8m sled kite up on Gun G/SP-013, while I was working a pile up on 20m. Expensive day out, never got it back. Must order another one.

73
Mickey

14

hi Andy,

I also carried out that calculation for the RF in the bleed resistor. It is not an issue as you say.

Even if it is a wire wound 1M resistor it will have zillions of turns and an inductance in the millihenry range (or higher) so is just like an RF choke, very high impedance, still no matter to the RF analysis I suggest.

But the static voltage on the wire is an unknown. 1kv through a 1 M resistor produces a 1mA current and the power in the resistor is 1 watt. But is the induced voltage only 1kv, or is it more or less? and once the resistor is connected, does that voltage get drained out of the antenna wire to zero, so the continuous current requirement is almost zero? or is the voltage across the resistor initially 1kv, then diminishes as the electrons find their way (up the wire) and then the voltage across the resistor is no longer 1kv but is much lower. it needs to be, otherwise the output circuit in your radio is subjected to 1kv initially. So bleed before connecting the radio…

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15

Hi,

i have had some experience with a hydrogen filled balloon and a longish wire. I have also seen up close, kite elevated antennas in action.

I have measured 80 m dipole voltages and currents when a thunderstorm was in the vicinity and also on dry windy days. I think I can add some practical comments.

The voltages induced by the build up of static electricity present in dry air or stray charges from nearby thunderstorms can be many 10’s of kV, but the total charge is small and hence available continuous current is 10’s of uA at most. If you get a mA then you are in a bad place at a bad time.

If you use a high ratio transformer at the end of the wire it likely has a dc path from the wire to the coax braid. If this is the situation - check with an ohm meter- then just earth the braid with a wire to a rod stuck in the ground. An earth resistance of even 100 ohms is fine. That will bleed away all the static charge quietly. No extra rf loss either.

Otherwise as has been said add a resistor of 100 k to 1 M ohm from the wire to a ground rod. Make a proper job with lugs, terminals or clips and a durable lead. This talk of losing the resistor on the mount suggests a very poor approach to a serious problem.

The resistor and ground rod need to be in place before raising the aerial unless electro-shock therapy is your thing.

Andrew you are right in that on some frequencies the resistor whether wire wound or not will appear inductive. At higher frequencies the shunt capacitance will become the main impedance contributor. I would use a common leaded component resistor. Just a few cents at your local supplier.

For field operation half a watt rating for the resistor is more than adequate.

Should you miscalculate the weather and there is a lightning stroke within a few hundred metres I suggest leaving the antenna connected and crawling away. Standing up next to such a setup invites trouble. Your life is of more value than the rig. It is possible the antenna will act as a lightning “arrestor” but it may also invite a big discharge.

Antennas elevated by these means will probably be most useful at the lower end of the HF spectrum. A conventional vertical or dipole will be more practical and more effective for 14 MHz and above for SOTA IMO. A near vertical wire that is many wavelengths long will fire most of its energy straight up. Even for wire angles down to 45 degees a wire more than a wavelength long might not radiate where you would want it to.

Some old timers used to wire a 10 k ohm half watt resistor across the feedpoint of their dipole so they could easily detect a break in the feeder. It also had the useful effect of bleeding charge on the “hot” side of the dipole to ground.

Good luck to all who go fly kites.

73
Ron
VK3AFW

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16

Best advice I have seen on this thread.

17

Thanks Richard, you are too kind.

Re the earthing link: The clip at one end needs to be able to fit over a BNC plug and at the other just big enough to fit over a medium sized tent peg. The wire could be automotive 5A wiring - pretty robust although 1 A rated hook-up wire would be OK too. A small piece of pcb material could be inserted to hold a 220 k ohm 0.5 w resistor with a bypass switch. The switch when closed makes the link suitable for directly earthing any form of portable station. That’s only going to be useful if there is some rf that needs to be shunted off.

I wonder if there would be a commercially viable demand for these?

73
Ron
VK3AFW

18

only thing with switches ron - is people forget to set them in their keeness to get on air - so i would just go for the KISS approch .lol - but thanks for the practical advice which has answered my question - all i need now is a day with reasonable wind thats not accompnied by rain - or just some breeze on the nicer days - not even got to test fly the kite yet - let alone worry about antennas … ho hum