View from Oban Bothy

View from Oban Bothy
Showing posts with label Aerials. Show all posts
Showing posts with label Aerials. Show all posts

Monday, 19 January 2015

Sunday 11th January, Hide and Seek

Playing Radio

To recap, for those who haven’t been paying attention, this is Hide and Seek with radios. Some poor unfortunate will hide themselves away in the area covered by a particular 1:50k OS map. Said unfortunate will be armed with a radio transmitter with which he or she will transmit from time to time. The trick is to find the hidden station, ideally before anyone else does. There can be any number of ‘hunters’, there’s typically 5 – 10 out hunting on a good day.

Following the huge success of the last Radio Direction Finding contest in December (I won / came first / beat everyone else in etc), it fell to me to arrange the next event in the series. Competing in these events is great fun, hiding is just as much fun – it just takes a good bit of planning….and it helps if you have an evil, sadistic streak   ;-)

The hiding place was to the east of Poynton, Cheshire. The exact location was at the edge of Prince’s Wood – the tall trees should have given me loads of opportunity to erect a decent aerial. The bad news was that I’d gone up to the site the previous day with my able support team of Eden & Nat but the bitterly cold strong wind-driven rain, hail and sleet made aerial erection very difficult indeed.

image Where we woz hidden

We managed to sling 130ft of wire up into the trees. The earth system consisted of a 4ft metal spike driven into the wet ground and a long section of barbed-wire fence. I tuned the aerial with my new toy:

The MFJ259C has proved it’s worth. Apart from making antenna tweeking a doddle it’s helped me learn more about aerials.

Like I said, we managed to get the aerial up but it would have been almost impossible without my eager assistants – thanks lads!

On the big day a total of eight teams took part, and along with their respective friends, relations, fans etc there was a total of 20 children of all ages in attendance. The children were mainly quite old – some were beyond retirement age.

My support team and I arrived on site in good time on the Sunday morning. Camouflage netting was set up, bacon butties were made (and eaten) and we were ready to go.

imageChris Heys at the start – photo lifted from the group’s Facebook page 

First transmission was 10am but we weren’t heard at the start. In these cases The Envelope is opened. The Envelope is only to be opened in cases of dire emergency – like when the transmitter can’t be heard at the start. This Envelope contains an approximate compass bearing to give the hunters half a clue of where to start looking. The start of the event, the place everyone meets up at to take the first compass bearing, was in Sale, Cheshire – about 10 miles from the transmitter.


Multiple transmissions are made, in theory to a sort of schedule. Unfortunately I screwed up the schedule….but only a bit.

My assistants, Nat & Eden, were lurking in the woods. They were armed with a walkie-talkie and were able to keep me up to date with news of approaching hunters. This kept them entertained for a good while. I’m sure they ‘assisted’ those trying to find the hidden station!

First man in was Dave Peacock, who charged up the hill in fine style – arriving around 11.10am, a good 3 minutes before the next man, Chris Plummer. The remaining hunters (apart from one team) arrived over the following half hour. The hunters approached from pretty well the same direction, all were pleasantly confused by the woodland. The DNF (Did Not Find) team managed to locate the apres-event pub. This was A Good Thing, a very pleasant lunch at the Boar’s Head in Poynton  rounded of the day quite nicely. 

Not being heard at the start was a bit of a pain – and a surprise. A huge and resonant aerial system should have been easily heard at a range of 10 miles – particularly when the transmitter was located in an elevated position. This has happened loads of times before, I reckon the transmitter’s knackered. I’ll bring my own next time – I know it works! 

Not much in the way of photographs I’m afraid. I took my camera with me but was so engrossed in remaining concealed that I didn’t get chance to use it.

Thursday, 24 January 2013

24th January, A low noise receive aerial

Modern electronic equipment is the curse of the radio enthusiast. The amount of interference generated by computers, television sets etc, is enough to flatten weak radio signals. The RF crud generated by normal household electronics is strong enough to completely wipe out radio reception. Even seemingly innocent kit can cause problems: some mobile phone battery chargers, some low energy lamps, the dreaded BT wireless hubs, the awful PLA/PLT 'data-over-mains' devices, switched mode power supplies, plasma TVs (these really are bad news), the list goes on and on.

For those who aren't radio enthusiasts but live in an urban environment, you can get an idea of how bad this interference is by tuning around on an AM radio, indoors, in the evening. You'll probably hear plenty of radio stations, but you'll also hear lots of whistles, heterodynes, buzzes, mushy noises etc, that shouldn't be there. Put your AM radio next to a (switched on!) PC or TV and I'd be surprised if you can hear ANY broadcast stations.

It's a legal requirement that electrical / electronic equipment shouldn't cause radio interference - but trying to enforce the law is far easier said than done. There's a huge amount of kit for sale on the high street today that's illegal - it just doesn't meet the RFI / EMC regs. Such is the influence and power of large corporations, they very often hide behind their product's 'CE' marking. I could go on about illegal 'CE' markings....but I wouldn't want to bore you.

This is all bad news for me, I've not been able to play radio for a while. I've experimented with different aerials with varying degrees of unsuccessfulness.

My first major attempt at quiet radio reception was this active, untuned loop. The circuit and notes are published on the website of Des, M0AYF - and very good it is too. Thanks Des!

This is the circuit diagram:

Loop amplifier circuit diagram.

The active loop aerial is situated about 35-40 ft down the garden, mounted at around 6ft above ground level.

It's certainly better than my 80m dipole on receive but it's still susceptible to picking up man-made noise - although at generally lower levels. I find it pays to switch between aerials to see which receives less noise - sometimes the dipole wins

My latest attempt at reducing received noise is this little piece of kit I put together this evening using bits from my junkbox:

This is the ENTIRE antenna - apart from it's power supply. The aerial bit is the plain bit of board on the left, the electronics on the right is a buffer amplifier that matches the extremely high impedance of the small aerial to the low impedance input of the receiver - it provides a significant amount of gain too.

It receives power down it's coaxial cable, and with a little bit of filtering at both ends, there is no need for batteries or separate power feed cables. 

It's tiny - and it seems to work quite well. Although received signals are down on the dipole, received noise levels are WELL down = much easier to copy weak signals that would otherwise be swamped by man-made interference.

It's an E-Field probe so it receives RF by capacitive coupling to the electric field rather than a magnetic field.
There is good evidence to suggest that the magnetic component of interfering domestic sources such as TV line time bases tends not to be significantly confined within a building whereas the electric field tends to be significantly attenuated by the structure. This can work to the advantage of this small aerial if it's mounted at a reasonable height and clear of mains wiring & buildings.
I can't claim any originality for this idea, it's the brainchild of Roelof Bakker, PA0RDT - quite a whizzkid by all accounts, and my thanks go to him.

Details of the aerial are widely published on t'interweb but I settled on the article published on the Crawley Amateur Radio Club's website and my thanks go to them also.

Tomorrow, time and weather permitting, the aerial will be mounted remotely down the garden.

Perhaps lower noise radio-activity will return to Timperley soon.

Wednesday, 12 September 2012

Wednesday 12th September, Getting the aerial up higher

Winter is approaching fast so I’ve been messing about with my aerial system at home.
The aerial is a half-wave dipole for 80m fed with twin feeder. My garden isn’t long enough to squeeze 132’ of wire in a straight line so the ends are folded down. The centre of the dipole was only at about 22’ agl, nowhere near high enough for the system to be an effective radiator. It performed adequately but that’s all.
Time for a re-think. I’ve managed to get the dipole centre up to around 30’, still not high enough but certainly better. This afternoon’s little job is to make a coax balun although the poor weather means it probably won’t get installed today.
The aerial is now certainly a better radiator on 80m & 40m, but receive noise is still a problem. Next door have a noisy TV, it may be plasma…the work of the devil!  Unfortunately my neighbours are unapproachable so I need to deal with the problem on my side of the fence. The RF noise next door’s TV generates makes 80m difficult to use in the evenings. I’ve treated myself to a noise canceller (MFJ1026)but I’ve not tried it out yet, I just hope it works!
image MFJ1026 Noise Canceller
This little box works by mixing signals from the main station aerial with signals from a ‘noise’ aerial – an aerial that is aligned to pick up the interfering noise. The phase and level of the interference is then altered by the box in a manner that allows it to cancel out the noise received by the main aerial = much reduced noise. That’s the theory, we’ll see.
My main interest is using low power (2-3 watts) CW (morse) on the 40 and 80m bands. Winter is a good time to play radio, hopefully my tweaked aerial system will pay dividends.
Apart from the noise canceller, my main station will remain unchanged: Yaesu FT817 transceiver fitted with a narrow CW filter, and a Bencher squeeze key. A ‘T’ Match ATU is used to optimise the match between transceiver and aerial system.
         image image
                        Bencher Key                                                   FT817
My FT817 has a microphone but it’s rarely connected – it just gets in the way!

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