I have just been playing with an antenna analyser that I borrowed from Mold and District Amateur Radio Club and finding out how the resonant frequency of a VHF beam alters with the height off the ground and its polarization. This is of particular interest to SOTA operations as getting the beam over 30 feet above the ground makes a massive difference compared to getting it 20 feet off the ground, even on top of a mountain. Turning the beam vertically alters its resonant frequency quite a lot and a beam tuned for 144 may be resonant outside the band (141-143 in my tests) when vertical. Mounting a vertical beam upside down made the antenna almost impossible to tune, which may explain problems some of us have had from time to time on summits. All fascinating stuff, as I am sure you will agree, but I don’t think 35 foot SOTA poles are going to catch on somehow and I haven’t added one to my Christmas list. However if Mr Clause is reading this I would love my own antenna analyser, please.
If you could have something radio or SOTA related for Christmas what would it be and why?
Please keep it sub £500 no Pro IIIs or “My own mountain”.
In reply to GW7AAV: I’ve had an MFJ analyser for a couple of years and wouldn’t be without it. I found the same as you when building a 3-el YAGI for 2M. I ended up deciding on a practical height and adjusting the (Gama-match fed) antenna accordingly for ssb, and using a wire J-Pole for FM.
And for Christmas : Due to the various heart medications that I’m on : warm feet and warm fingers!! and the will to keep practicing CW until I’m confident to use it in anger.
Seasons greetings to all activators and chasers AND our very tolerant partners and companions
73s DE Dave (G6DTN and M0DFA)
PS Will be in the SE area towards the end of the week. Visiting Mother-in-Law with XYL, but hope to be allowed out perhaps on Thursday.
A 20m vertical antenna for SOTA.
An abundance of good weather and family good health allowing for lots of expeditions in the winter bonus season.
SOTA associations in Italy, Spain, Croatia, Portugal, Luxembourg, San Marino, Slovakia and Andorra.
More SOTA associations and activity in North America.
That elusive 200th activator unique.
A curry.
Macc to beat Everton and get into the 4th round.
I’m asking a lot. Maybe I’ll have to sell my soul to Santa.
In reply to GW7AAV:
If the tuning of the beam changes when you change to vertical polarisation it suggests to me that either the coax is running parallel and too close to half the dipole, or the fibreglass pole has some carbon fibre in it! I would try running the coax to behind the reflector before taking it down to the rig. The effect of the ground gets interesting on a hill. On a hill with expanses of rock exposed the true earth corresponding to the water table will probably be several metres or even tens of metres below the surface, whereas a summit with peaty soil will have high conductivity at ground level. Another consideration worth experimenting with is tilting the beam up or down relative to the ground.
My Xmas wish list:
The XYL gets her knees fixed so that we can do more hills together.
More sunspots
An FT817.
More sunspots.
A SOTA Association in the antipodes - late night chasing, yummy! Also have the Americans, Canadians etc get the bug - evening chasing, too!
MORE SUNSPOTS!!!
That’s a strange one Steve. I would have thought that an antenna once tuned to a set qrg, stays on that qrg and the effect you were seeing was a difference in VSWR due to antenna orientation, which in turn fools the analyser into seeing a different resonant frequency for the antenna.
For Christmas…
My 3,000th unique summit chased and 25,000 chaser points.
If the tuning of the beam changes when you change to vertical
polarisation it suggests to me that either the coax is running
parallel and too close to half the dipole, or the fibreglass pole has
some carbon fibre in it!
Probably both, with the two antennas I was playing with (a SOTA type beam and a HB9CV) both are difficult to avoid the coax interacting and the poles probably have some carbon fibre as you say. What interested me was the fact that the antennas remained resonant but the frequency that they were resonant on changed, particularly in relation to the height above ground. I had observed previously that antennas I constructed where the best SWR had been say 1.5-1 at ground level had much a much better SWR once up on a mast, but I have never used an analyser before or been able to see that a beam centred on 144.300 at ground is actually better at say 144.250 or 144.350 once on the mast. Even if these differences are small we need all the help we can working low power signals from hilltops and picking the exact frequency your antenna is resonant on may help you decide were is best for you to QSY to.
I love to make antennas and the subject is a big fascination to me, however most of what is written gives me a headache reading it. Learning by doing is another thing entirely and I would have oodles of fun with my own analyser.
I have to agree on the sun spots.
Steve GW7AAV
In reply to GW0DSP:
Those tickets to DL wouldn’t go a miss here either.
Let’s hope we will both be celebrating on NP Fun Day then Mike! Last time I saw us play Everton at the Moss, we played their star-studded (but ageing) team (Ratcliffe, Southall, Van den Howe, Sharp et al) off the park and won 3-1 in a friendly (late 80s). A similar result on the 3rd, followed by a good day and 9 points for NP-007 will make a perfect New Year weekend. Bring it on!
In reply to GW7AAV:
I used to use an HB9CV for 2 m portable years ago, and use one for 6 metres now. When tuned correctly they are good antennas but they are the very devil to keep in tune! The phasing section between the two halfs flaps about and any movement detunes the antenna. When it rains, water on the boom detunes the antenna. In hot or cold weather the tuning changes - it probably needs a zero coefficient trimmer! I tape the coax to the boom and run it to the end of the boom extension, when it hung free gusts of wind moved it and - you’ve guessed it - detuned the antenna! You tend not to find things like that in the books!
Thinking about it, I wouldn’t mind finding an analyser under my tree Xmas morning!
For a 5m length of RG58C used on 144MHz and fed with 5W the additional SWR loss going from 1.5:1 to 2:1 accounts for only 120mW. Even going to 3:1 the extra loss is only 390mW.
However, the reduction in loss in replacing RG58C with Times LMR-200 at that frequency is worth 500mW less loss.
So you can can faff about trying to figure out how to produce a portable VHF antenna which suffers the minimum in SWR changes when exposed to a varying environment (hard) or just replace the wet string with suitable cable (easy).
Which shows that a badly matched load and low loss cable is better than a perfect load and rubbish (for the frequency in use) cable. Or to use a loathsome management expression: pick the low hanging fruit first!
Those tables look impressive at first sight, Andy, but comparing the two 5m cables at any given SWR, whats the difference in dB? Corresponding to what difference in S points? Vanishingly small, I reckon!
Before getting the stepladder out, management take account of the law of diminishing returns!
Indeed. Which is my point. I doubt you’d really notice the difference. However, optimising something hard when you could optimise something easy doesn’t seem right to me other than the intellectual satisfaction it produces.
The tables show that there is no practical advantage to fixing the varying SWR as the better cable produces a superior solution than the perfect antenna with standard cable. Of course, if you are using a better cable like LMR-200 already then you may be tempted to work on the antenna. But probably moving to LMR-400 will be better still! And after that…
Well in Steve’s case he is lucky in that his family accompany him on activations so he has a team of captive sherpas. He’d probably be better to get all of his family to each carry a 1m lenth of LDF-750 fitted with N connectors. They could assemble a 5m mast made entirely of coax which would have the ability to support the antenna with the minimum of losses. It’s unlikely to bend in the wind and the loss would be of the order of 0.04dB.
Calculating the actual power lost in the cable is left as an exercise for the reader along with deciding where I stopped being serious!
We wish that we have more summits in Belgium, more than the next 22 … maybe around 200 or something.
It would be wonderful for us and ALL Belgian radioamateurs, maybe there will be more “SOTA” activity in Belgium.
