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Brixham station - Elecraft KX3 and Expert 1.3K-FA

Sometimes in London, Sometimes in Brixham, Devon

Most of my operation is portable. I enjoy the challenge of getting big station performance out of a modest portable setup by carefully choosing the location (often best right next to the sea) and using kites, helium balloons, and tall trees to deploy large temporary HF aerials.

All my equipment has been chosen with portability in mind - for HF I have an Elecraft KX3, a KX2 and a selection of amplifiers. For VHF/UHF I have a 2m transverter fitted to my KX3 and I also have a Yaesu VX-8 triband handheld which I've recently been using for QSOs via the Amsat FM satellites.

Yaesu VX-8 and Arrow handheld dual-band yagi for FM Satellite operation




I had fun being the DX as 8Q7PW in the Maldives this Summer, usually I'm the one chasing the DX though. My favorite antenna for portable DX is an end fed half wave vertical (EFHW), this antenna is easy to deploy and (unlike a 1/4 wave vertical) works well with a very minimal ground plane due to its high impedance.

My favorite portable DX location is Brixham breakwater (below) - the proximity to salt water increases the efficiency of the antenna significantly and also results in a very low angle of radiation. 20 meters is usually full of 20db over S9 signals from this location! For 14Mhz and above, I use a 12m mast to support my EFHW antenna, this mast can also be used to support a 1/4wave vertical for 40 meters.




What amateur would not like a head mounted 12m mast? Mast with 1/2 wave endfed for 20m on Brixham breakwater. KX2 with 200W mobile amplifier and 16Ah battery.

Low band DX:

For DX on 80m and 40m, my preferred system for supporting a vertical EFHW is a helium balloon, mostly for the ease of deploying a vertical antenna of the size needed for the longer wavelengths. The problem with this system is that it only works well on (rare) days that are completely wind-free, but operated next to the sea this DX setup works incredibly well if the weather conditions are right!

 Balloon portable station ready to roll! 1/2 wave end fed vertical antenna for 80m deployed on Brixham breakwater. 


The computer simulation on the right illustrates the huge advantage obtainable from portable operation over salt water, particularly on the low bands. The dotted line shows the radiation pattern of a 3 element Yagi for the 80m band at a height of 40 meters over a normal ground (this is a huge antenna, at a significant height). The red line represents a simple vertical EFHW antenna (such as shown above) operating from ground level over salt water. The Yagi only has about 1.5dB of gain over the EFHW, and this occurs at an angle of about 30 degrees, which is useless for DX. At a more DX useful angle of 5 degrees, the EFHW has about 8dB of gain over the Yagi! The Yagi, however, does have the receive advantage of rejecting noise and signals from unwanted directions, my KX2 receiver does sometimes struggle with all the huge signals the EFHW provides although the omnidirectionality can be fun, calling CQ one evening on 40 metres SSB I worked 5 out of 6 continents in under 20 minutes using a balloon supported EFHW on Brixham breakwater!

An EFHW antenna has a very high feed point impedance (around 3KΩ) so needs a matching transformer. Here's one I made, which is good for 200W.  The matching unit uses a FT240-43 toroid with 2*7 secondary turns, 2 (bifilar wound) primary turns and a 100pF capacitor across the primary. As can be seen, it gives a really great broadband match from 80m all the way up to 10m!

Matching unit detail. Input SWR and |Z| when feeding a 3.3KΩ resistor.

Measured |Z| of 20m EFHW (about 2.2KΩ at resonance).


London HF:

In central London, a vertical EFHW mounted at ground level is completely unusable due to extremely high noise levels. Since man-made noise comes from low angles (no more than traffic and building height) and since there's unlikely to be much useable low angle signal finding it's way through all the buildings, I use an NVIS antenna (a low height horizontal dipole with a high radiation angle) when operating portable from my local park in London. The noise reduction is significant, on 40m with pre-amp off, the dipole noise levels are a manageable S3 compared to an impossible S9+ on a vertical EFHW. I believe this is mostly due to the difference in radiation angle, although the directionality of a horizontal dipole gives a further noise reduction. Note that at high angles of radiation, a horizontal dipole is almost omnidirectional so for NVIS work, orientation can be chosen purely to null out the worst source of low-angle noise (one of the ends of the dipole should be pointing towards it). 

I use a catapult with a fishing reel attached to launch a weight and fishing line over a tree branch about 8m up, the coax is then tied to the fishing line and pulled up and over the branch and used to support the center of the dipole. The two dipole elements can then be attached to low branches at the edge of the tree's canopy, keeping the entire antenna well above ground and (hidden inside the tree) protected from other users of the park - it takes no more than 5 minutes to deploy the antenna! The high radiation angle of this antenna makes it weak for DX but great for NVIS rag-chewing with UK and near Europe, which is mostly what I use it for - a case of making the best of what the situation in central London has to offer in practice! (For more info on NVIS, check out this excellent reference on NVIS antenna design.)

Brunswick Square gardens, Bloomsbury. Right in the center of London, the #1 problem of this portable location is the incredibly high noise level. Computer simulation of a low height horizontal dipole shows a high angle of radiation, good for NVIS and excluding low angle noise. A balun at the dipole center ensures the (vertical) RG174 feeder won't pick up unwanted noise.
KX2 and 40W amplifier ready to go. The small 7Ah LiFePO battery under the amplifier provides all day operation with this setup. Home made antenna launching device. Dipole for 40m installed inside tree canopy (coax marked in yellow, dipole elements highlighted red).


More on Balloons:

Again on Brixham breakwater, my first attempt at balloon DX on 80m using a 1/4wave ground plane antenna and just 50W netted a contact with J69CU in St Lucia and a contact with ZL2OK in New Zealand (the antipode of the UK).


A 5:30 am selfie. Next time I try night time portable operation, I'll remember to bring a torch (oops?). Still, the balloon has gone up in the dark and signals are coming in! Dawn is breaking, and I have J69CU from St Lucia in the log! Time to see where that balloon I released in the dark went.... Not so good - a slight breeze has come up and as you can see my vertical is now a horizontal. :-/
Hurrah, the wind has gone and my vertical is once again vertical! And there's still an hour or two of grey-line propagation left.... 7:29 am - 80m balloon DX hole in one! David ZL2OK over in New Zealand, which is the antipode of the UK.  10:20 am - By this time I'm feeling truly dreadful after my unpleasant 5am awakening and a cold 5 hours sitting outside in winter, but extremely pleased with the results. Time for a few big cups of celebratory coffee ;-)


Technical info on the balloon and antenna setup

Helium isn't cheap, especially if bought in small disposable bottles intended for party balloons. I spent some time researching the best materials for an antenna and tether that would be strong and with good electrical characteristics but at the same time extremely lightweight, thereby not requiring a large balloon to lift.

The antenna wire used was 0.38mm copper-clad aluminum enameled wire. From my very rough calculations, this wire would handle my 50W and not waste too much power in resistive losses. Bear in mind if you want to transmit more power you might need thicker wire, which might, in turn necessitate a larger balloon. Aluminum gives a much better DC conductivity / weight ratio than solid copper, so is preferable in a situation like this when weight is at an extreme premium. In addition, due to the skin effect, the copper cladding combined with the larger radius of an aluminum wire means a much better RF conductor than if solid copper wire were used, the copper cladding also means that it's possible to solder the aluminum wire.

The antenna wire itself should not be used to tether the balloon, in addition to the fact it has insufficient strength, there is the danger that if the wire breaks, the balloon will fly off potentially trailing a long length of conductor which could be extremely hazardous anywhere near power lines. A bit of research turned up a supplier of kevlar thread on Amazon, the thread is extremely strong and adds negligible weight to the antenna. I created a line formed from the kevlar thread attached to the wire element every few feet, with a few meters of kevlar thread between the end of the antenna and the balloon (I've read reports of antennas terminated at the balloon popping the balloon on transmit).

It is also worth highlighting here that balloon (and kite) antenna can be very dangerous due to static buildup in the antenna wire, and should never be operated without some way of bleeding off this charge to ground. I used a 100K resistor between the antenna and ground, a resistor of this value has no effect on TX/RX signal strength and saves my equipment (and me!) from undesirable static induced death ;-)

The balloon chosen was 24" in size, it was calculated that this would give about 95g lift using 80% helium "balloon gas". Since the balloon weighs 28g and the wire+kevlar thread weighs 10g, this leaves 57g spare lift (or 0.6 spare lifting capacity). The balloon size chosen was probably about right, although if it was perfect calm, a smaller balloon would be fine! In anything other than a flat calm, spare lifting capacity does not go to waste, since spare lifting capacity translates into a lower angle of lean in a breeze. Regardless of surplus lifting capacity, in any breeze much above 5 mph a balloon antenna will be tending towards the horizontal due to the wind resistance of the balloon itself.

Due to its small molecule size, helium has a habit of leaking out of latex balloons quite quickly, meaning it's usually only possible to get a few hours of good lifting service from them. I treated my balloon with a product called "hi-float" which is supposed to overcome this problem, if weather conditions permit in the next day or so I hope to re-use the balloon, which I have saved! (Note: the stuff seems to work, the balloon has lost virtually no lifting capacity in the 30 hours since inflation.) 

The 80m 1/4 vertical went up very easily, considering the small 10g contribution to the weight budget from this antenna, I feel that a future attempt would definitely benefit from a 1/2 or 5/8 wave vertical.



I also sometimes use a kite, the advantage over a balloon is that a kite has much greater lifting power, although the wind tends to vary in strength and direction over time, making it quite hard to engineer an antenna that's reliably vertical for DX.


Magloop in central London park



8411686 Last modified: 2017-10-26 08:48:11, 20098 bytes

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