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Dipole height and reality

Dear friends,

Can you experts help a beginner please - I know the “as high as possible” or “up a quarter - third wavelength” rules for getting a dipole up, but what have folks had success with in reality? I understand how a low dipole becomes an NVIS, which might not suit my purpose. Or would that NVIS setup be more what I want?

I’m interested in 40m / 20m. I have an Army surplus sectional mast that is about 37’, but lugging that in a duffle bag doesn’t seem in the cards. The painters pole at a store in town is 18’, but pretty unweildy.

I’m in Central PA (W3/SV), where it is somewhat hilly, and much of the trees are pine or hemlock with dense lower branches. I just don’t see me throwing that dipole up more than about 20-25 feet or so.

Thanks for any/all suggestions,

Eric
KB3UYT

In reply to KB3UYT:

Most activators use telescopic fibreglass/carbon fibre fishing poles like these http://www.sotabeams.co.uk/SOTAPole.htm You can get them from ebay in various lengths.

For getting the ends up higher some people use a catapult(slingshot) to throw the line over a tree branch.

Colin G8TMV

In reply to KB3UYT:

Try going to “Full List of Discussion Topics” above and search on Dipole and Inverted V.

Get the middle of the dipole up high on a pole (inverted V) to get the current peaks up there, but also keep the dipole ends off the ground (since they get the voltage peaks) using string or thin garden canes or both.

73
John GM8OTI

In reply to GM8OTI:
Lot of QRPers us this pole with inv Vees with great success.

http://www.jackite.com/product_info.php?cPath=41_44&products_id=61&osCsid=0fe629c4f92af254c608a7a757fe8386

I have a 28’ and its a great pole. I use HB yoyo dipole.

In reply to KB3UYT:

There is no simple answer, Eric, it all depends on what you want out of your radio.

On 20 metres and 40 metres, a 20 to 25 foot high dipole will be a quite useful antenna. Few SOTA activators have their dipoles higher than that, if as high. In Europe that will reliably get you contacts throughout the continent on 40 metres when the band is playing ball, and the USA is on a similar scale so you should get contacts throughout the States. 20 metres gets you further afield but of course you will rarely get the closer stations. When I say further afield, I mean that in reasonable conditions 5 watts will get you contacts from this side of the Atlantic, and I am confident about this because I have worked into the States on 20 metres with a 20’ high doublet and an FT817.

If you want more DX, then I would suggest that you think in terms of a vertical, a 10 metre GRP but not carbon fibre fishing pole and a wire vertical on it with radials, or a Rybakov 806, but this reduces your capability for NVIS contacts on 40, which for SOTA would probably be your bread-and-butter contacts.

I believe many of the higher summits in your area are “balds” but the lesser tree-covered summits may give you problems because of signal absorption in the trees, this may affect vertically polarised signals more than horizontally so I suggest that you be prepared to experiment and see what works best. In fact that is the best way to procede: ask advice, read books, then experiment.

73

Brian G8ADD

In reply to KB3UYT:
If you do have trees to work with, try finding a fist-sized rock, fairly round. Tie a light cord to it using a scaffold knot (Google it if that’s unfamiliar to you) and anchor the other end to something like your backpack. Checking to make sure you have clear space, twirl the rock in a vertical circle, David-sling style and release the rock on its upward trajectory, aiming to lob it well above a tree branch of your choice. I normally wear fingerless leather gloves when I’m hiking, and I certainly want to be wearing them when I’m lobbing. A 50-foot lob should be a cinch. On a good day you can double that height without much muscle power. The reason for that pretty substantial rock now becomes apparent, because the next challenge is for the rock to pull that cord all the way down at least to arm’s reach from the ground. Lighter rocks seldom do the job. Untie the rock and fasten the center connector of your dipole and carefully launch your antenna while tending the end wires through tree branches. Tie those ends to lower branches or even lob them gently over branches using that same rock. I pre-tie the ends of my dipoles to their own light cords using a Hunter Bend, saving the weight of end insulators. (Google if necessary.) I use orange cords to make finding them easier.
73, Bruce N7RR

All the summits I’ve been on require you to take your own tree. :wink:

  • Phil G3YPQ

In reply to KB3UYT:

Can you experts help a beginner please - I know the “as high as
possible” or “up a quarter - third wavelength” rules

That model is for flat ground, which is rarely the case on mountain top. For VHF the radio horizon can be estimated quite simply from the altitude and you can have a look on ARRL handbook for this. The HF antenna pattern is not so simple to model. I have tried some models in EZNEC. One of them is a “cylindrical” mountain for example 400 m above the flat ground and diameter of 100 m. The other model is a mountain ridge 30 degree slope from one side and 60 degree on the other - this is close to typical operation is Alps. More complicated or realistic models you can not do with EZNEC. Related to this there is software from KD2BD http://www.qsl.net/kd2bd/splat.html .

73, Jaakko OH7BF/F5VGL

Edit: EZNEC does not understand diffraction or other “geometrical optics” effects

In reply to KB3UYT:

http://www.qsl.net/aa3rl/ant2.html

The figures quoted are for 40 metres, but can be extrapolated for any band.

Bob G6LUG

In reply to KB3UYT:

Hi Eric

There is a simple solution which would cover your requirements.

I occasionally use a full 1/4 wave vertical supported by a 10m (33") collapsible fishing pole. An antenna with a length of around 10m just happens to also be 1/2 wave at 20m and 5/8 wave at 17m which I use simple matching techniques to resonate (one day I’ll write an article about it).

I’ve found that 40m is the cross over point where verticals and low slung dipoles work well portable. From my experience above 7 Mhz verticals appear to out perform dipoles that are mounted in inverted V configuration with their apex around 4-6m agl (usual practical height for portable use).

Carolyn (G6WRW)

In reply to VA2SG:

That looks great. Especially since it folds to 46". Thanks for the link.

In reply to G6LUG:
Those numbers seem to support the NVIS idea. Maybe it wouldn’t be so bad only .1 wave up, with that 8.21 gain. The Resonent freq is low, but it might tune up.
Thanks!

In reply to G6WRW:
That seems like a good idea. If you get the article together, I’d love to see it. Thank you!

Thank you to everyone who responded.

I’ll be doing a dry run at Scout Camp this week (non-SOTA, but in the mountains), and we’ll see how things go.

Thanks again!

Eric
KB3UYT