For your wire dipoles, do you worry about what apex angle it’s designed/tuned for?
90° is allegedly the best from my researching, but clearly that cannot always be achieved, especially on the longer bands and shorter portable masts.
On this thread:
I found this interesting point from Andy:
So it would sound like taking a hit on wider angle over close to ground is preferable (and makes sense)
And clearly changing the angle changes the designed resonant length. How much of a variation would you see in practice?
I’m trying to cater for 2 mounting heights, and then of course the unknown variable that is on site location…
I was planning on a linked dipole for 20m, 30m and 40m. Does that sound sensible?
My thinking being that the 30m dipole should also work hopefully on 15m without a tuner.
Don’t worry yourself about trivia. You are trying to apply home station standards to a portable environment. The odd db you will loose or gain because of the angle of the apex, or the height of the ends etc is not worth the worry over.
Do what Andy suggests just make a linked dipole, don’t worry if the measurements are a few mill out her or there. Stick it on a fishing pole and plug it into your radio. All of a sudden you will see there is no point asking too many questions.
I use a walking pole in a similar manner. One end of the dipole however usually ends up at ground level (the feed point).
Sometimes both ends are virtually at ground level. I have nothing to measure differences between end height and apex angle. That would be a job for an antenna range and loads of instruments…
On 60m I dispense with the pole completely to give a completely flat top, or at least as flat as the ground the wire is lying on :-). Dead simple and quick to deploy, but you need to be careful that sheep don’t trip over or eat it.
Seems to work just fine.
Angle of an inverted vee is not important in SOTA activating, obviously the more horizontally shallow the angle of the dipole the better, just like at the home station. Portable I’ve used a low 5m pole with a 40m dipole lots of times with the ends of it within a few feet of the ground - use longer thin cord to get the maximum splay if this bothers you, but then there is more windage and you put greater stress on your pole and it may snap!
The most important thing is to get spotted on SOTAWatch and the chasers will be there, whatever your power and equipment is, within reason. So get a smartphone or phone a friend who will put you a spot on. This is more important if you are QRP on SSB. On CW chasers sit on certain well known frequencies and the RBN can also find you as well, the RBN network also works for DATA signals as we saw yesterday when DJ6TB/P came up on RTTY on 14089.
I have even had SOTA contacts when the antenna has blown over and was lying on the ground so the answer to your question is not to worry - just get on the hill, spend 10-15 minutes erecting a simple antenna and your station and then sit back and enjoy the experience of between 10 and 20 people calling you all at the same time desperate for a contact.
Having not done any HF for SOTA I feel it hard to justify saying anything! But having built the Antenna that Andy recommends (Credits to John GW4BVE) for publishing his design and then using it on a beach or in a field, it works really well.
Steve you can discuss the various points on here, but trust me, the best way to learn about SOTA is to have a go!!!
Get yourself down the Ross Road from Hereford an take the easy stroll up Aconbury Hill.
You’ll learn far more from that than you ever will on this reflector.
LOL. I know what you are saying
I am at the mercy of deliveries and scarce free time.
I do like to research and plan ahead while waiting for goodies though, this is true.
I have taken on board a lot of the good advice here, pole, link dipole over trap dipole, ground auger stake thingy, batteries, Anderson powerpoles, etc, all new things for me.
Even an FT-817 though that was a stroke of good fortune (still plan on FT-857 as first choice) My previous ‘life’ was all heavy duty stuff and VHF.
I have a reel of coax (for portable and other projects) and some connectors arriving today (according to tracking).
Once I have made my dipole I should be ready to roll. I think I have nearly everything now.
I followed the instructions on the SMP site re setting up a SOTA beams link dipole . Richard made it of course but I used the instruction Re centre height, in my case 8m and have enough tie off cord to have the ends about 1.5 m off the ground . Which is slightly higher than this short man who can walk under the end. I need to drop the Squidy 3 sections to change the links between bands. I leave the strings attached when I wind it up. Works good .
ian vk5cz …
I hope I can add some value to this discussion.
A dipole has a feed point impedance of 72ish Ohms (72+j0 at resonance). When configured as an inverted Vee, the sharper the included angle the lower will be the feed point impedance (50+j0 would be good). The nearer it is to the ground the lower the feed point impedance. Laid on a conducting mat the feed point impedance would be zero.
The resonant frequency of a dipole is principally determined by the length of wire (some end capacitance will lower the frequency).The included angle of an inverted Vee as only a small effect on resonance.
It is the Current in the aerial that creates the Electromagnetic Wave, not the Voltage. So an elevated feed point is desirable. It is also why a dimensionally small magnetic loop still functions and we can shorten the elements of a mini beam with capacitive top hats with only limited effect on transmission.
An inverted Vee for 80m through to 30m is likely to be electrically close to the ground. It illuminates the sky (essentially Omni-directionally), not the horizon. During daylight it will provide coms. with one reflection - range up to 1000miles ish
To work stations beyond 1000miles we need to illuminate the horizon. This is best done with a vertical radiator. A ground plane (GP) aerial is practical but with the feed point on the ground and ground mounted radials we have a compromise and ohmic losses.
One solution is to elevate the feed point. I use a vertical for 20m where the mast is 10m high, the feed point is at 5m with the radiator above that and 3 radials attached to the coax screen at the feed point, The Impedance of a true GP is 36Ohms, if however we are sneaky and slope the radials, each 5m long (and extended with nylon cord to support the mast) we can achieve a 50 Ohm (50+j0) match . This approach is equally suited for the higher HF bands.
I always test out my aerials with an aerial analyser. I can usually get an VSWR match of better than 1.5:1 across the required frequency range. In my experience the only exception is 60m. To operate on 5.260 CW and 5.389.5 SSB I add “tails” on the end of the dipoles to lower the resonant frequency for CW. I do not use an AMU across 60m to 20m.
On the activation, things are never the same as on the test field, but we do the best we can in the circumstances. We all operate at times on the edge of signal readability, so every little helps. I occasionally check the SWR using the FT857, I never see it displaying more than one bar on the display unless I have set something up incorrectly. Finally, keep people and animals away from the dipole ends. At 100W input the voltage on the ends is about 5000V peak.