Question for inverted V design

Looking at an inverted V dipole for SOTA, now here is the question, what is the best angle of the legs for optimum omnidirectional radiating? Sean M0GIA

http://www.k7mem.150m.com/Electronic_Notebook/antennas/inverted_vee.html

It would appear that between 70 and 110 degrees is OK with 90 degrees the best

Peter
G1FOA

I use a wire dipole with about 2x7.5m on a 8m telescopic fibreglass mast for most of my SOTA activiations. Depending on local circumstances I have to use a top angle of anything from 120 to about 90 degree. If I achieve less than 90 deg I would extend the mast to less than its maximum height. This happens occassionaly on step peaks were the outer ends of the dipole are lower than the foot of the mast.

I have never noticed any pattern in reception. The antenna simulation gurus might show us otherwise. I would not worry about obtaining omnidirectional pattern.

73 Heinz

You could download one of the many antenna modelling programs from the myriad available and experiment. Then compare the predicted results against the experimentally observed results. Adjust the ground model and lather, rinse, repeat!

My fishing pole is 5m and that seems to work as well as people who use 7m poles certainly on 40m/60m. I don’t do enough 80m operations to have any meaningful data.

Is it omnidirectional? Sometimes.
Does it work better when there’s a fence underneath? Sometimes.

The only thing that seems consistent is that when the ends of the dipole are near the ground its performance is much, much worse than when they are higher.

Andy
MM0FMF

Thanks for the replys and the link, i knew i could get some feedback on this type of antenna in the real world. Sean M0GIA

I have never noticed any pattern in reception. The antenna simulation
gurus might show us otherwise. I would not worry about obtaining
omnidirectional pattern.

The *NEC antenna models usually use some kind of flat ground approximation. Most of the activators already know that this can not be realistic case on a typical rocky summit at 2500 m. I would guess that a free space model is closer to truth in this case, though there can be some diffraction from the nearby summits. So how does a dipole radiate in free space? Ok, lets assume that small part of the energy radiates directly above like in a good NVIS antenna. So do we have effectively a combination of two kind of antennas? One in “free space” and an other one much less efficient on flat ground?

73, Jaakko OH7BF/F5VGL

I agree Jaakko that while it is ok to model antennas in CAD programmes like NEC once out in the real world things are different. This is how i came to use more than one radial for a HF vertical to work on as a monoband vertical and give the same SWR reading over differing grounds, this is how the Magic Moggy was born (patent pending) and thanks Tom for the unique name!

I am looking at stability over diferrent ground and of course not having to use an ATU. Sean M0GIA

In my experiments with verticals (mostly in a quiet /P location near sea level rather than at the top of a hill) I have used radials as described in the RSGB “Radio Communication Handbook”, 5th edition. On pages 12.81 - 12.82 it is noted that a solution to matching can be found by tilting the radials downwards, to raise the feed point impedance, and if the angle is 45 degrees the system will be a good match to 50 ohms. That’s what I aim to do and it seems to work well. It suits me nicely, since I don’t want to mess about with an ATU - if the power appears not to be reflected back to the rig, as with an inverted V, then I’m winning.

What I do notice is that the noise using a vertical has different characteristics to that using the inverted V. I prefer the V on hilltops since it is quicker to set up.

73
John GM8OTI

It comes down to what you want do once on summit, in my own experiments verticals work the DX from height simply down to the fact the RF see’s the horizon, inverted V type dipole’s are the choice for me or at least how i see it for certain bands and as you say ease of setting up. The experiments continue! Sean M0GIA

The way I see it is the best angle is the one you can get. While 90 degrees might be seen as optimum it is almost impractical on 80m from a summit unless you want to carry a mast nearly 16m tall. I estimate my portable 80-10m 'inverted vee’linked dipole to be mounted at about 117 degrees and as a result probably performs more like a low mounted horizontal dipole than a true inverted vee. My 60-10m fairs no better with an estimated 124 degree angle but with this one I could use a larger 10m mast and get the legs near to 45 degrees. I do own one or two 10m masts but I choose to carry one of the lighter 6, 7 or 8m ones.

Every antenna is a compromise and what is best is what works for you.

Regards Steve GW7AAV

I cut a dipole 60’6" each side,fed it with a random length (18’) of 300 ohm ladder line. I use my MFJ 971 atu and Yaesu FT-817ND therefore only 5W. A fellow ham recommended 15’ off the ground in the centre and 2’ off the ground at the ends. This clearly takes advantage of NVIS and a test on my local football pitch using this configuration on 80m gave me genuine 59 reports from England, Scotland and Ireland from several WAB chasers.

This antenna is ideal for UK contacts though if it’s DX your chasing then as far as I am aware the higher the better.

Hope this helps.

73 Chris 2E0FSR

I agree with most of what has been said above. Don’t worry too much about the angle in the vertical plane and don’t forget about the angle as seen from above.

With my linked dipole I found a center height of 4m, enough spare rope at the ends to get the dipole ends at least 0.5m off the ground, and an angle (when seen from above) of approx 120 degrees works a treat. The 120 degree angle drops the feed point impedance down to something nearer to 50 ohms and in conjunction with a suitable ‘back stay’ makes for a stable structure in all but the highest winds.

I’d recommend borrowing an antenna analyser and having a play around in the park, with a friend to move the ends around as you take notes etc. That said, there’s no point getting obsessed with getting the SWR spot on as you’ll never be able to put the antenna back up in exactly the same configuration again anyway.

Getting the ends off the ground seems to be the key to getting a good SWR. I note however that John YSS has on occasion had passable results with his 160m dipole lying on the ground! Rock, even if damp, seems to make a poor earth. The antenna books and simulators all lead one to the view that a low (horizontal) dipole will act as an omni-dimensional radiator with a good proportion of the RF going in an upwards direction. This is, of course, generally what one wants for inter UK/EU working. DX is a different matter.

I think the key thing is not to spend too much time worrying about the precise dimensions but to get out in the field and get a feel for what is practical.

73

Rick.

Hi Sean

http://www.moosedata.com/g6wrw/hfdipole.pdf

(thank you Andy for the host)

The end of the article has references.

The angles are not as critical as you would think when the antenna is used on hill tops and the only time I have problems is when the ends of the 80m section are closer than 1m above the ground.

Any low dipole (regardless of apex angle) will exhibit an omni pattern when mounted close to the ground in relation to its designed frequency.

What is more important (and is over looked) is vSWR and bandwidth which can be more critical with low antennas which is dependant on the diameter of the wire used for the radiating elements.

Carolyn

Sean,

Richard G3CWI wrote one of his Radcom “Portable” columns all about this a while back. The article included the formulae and constraints you seek about heights, angles etc.

I periodically take my piles of radio magazine back issues to the Medical Centre on Sunderland Street and scatter them around the several doctors surgery waiting rooms in there. If you have a rummage around there, you never know! Or maybe Richard still has the original on his hard drive if you ask him nicely…

I agree with Carolyn’s observations above FWIW.

Tom M1EYP

Thanks Tom, i will ask Richard if he still has the file/doc, i am thankful for all the replys and links and advice which has steered me closer to a final build. Sean M0GIA

I have never used HF from a SOTA as I have a strong preference to VHF/UHF.

But if anyone is interested maybe we could organise a ‘40M/80M SOTA Fun Day’.

A perfect chance to test those antennae for inter UK working.

73 Chris 2E0FSR

Great idea, Chris, but how about making it a 40/60/80 Fun Day…or now the sunspots are showing a little life, an HF Fun Day?

73

I may be mistaken but isn’t 60M for full licence holders only?

And conditions permitting HF would give worldwide coverage. 80M/40M NVIS would give UK coverage.

HF from SOTA is not my cup of tea but as I’ve had lots of support for the VHF Fun Days it’s only fair that I give it a go. I’m not the type Of guy that takes his bat & ball home if you don’t play my game.

73 Chris 2E0FSR

I may be mistaken but isn’t 60M for full licence holders only?

In the UK yes. However, there are other countries which also have 60m allocations and SOTA associations where the rules are different.

Andy
MM0FMF