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NVIS Antenna for SOTA


#1

Having used 60m from just a few summits in the last couple of years with my ‘linked’ dipole (which covers 10m,20m,40m,60m) I have until now mainly used it with the center at 5-7m AGL (depending which of Matt’s 2E0MDJ poles I have borrowed) and the ends just off the ground.

At home my only HF Antenna is a 40m home-made ‘windom’ style with the high point around 9m (near the Chimney) and the ends around 4-5m AGL - this I find very noisy on all the HF bands, but I’m not entirely surprised by that being in a built-up area.

I am wondering if next time I go /P about using the dipole supported just a meter or less off the ground between a couple of walking poles.

I know Pete G4ISJ is hoping to do some 60m activity from a summit using a similarly low (or perhaps even on the ground) antenna, so I suspect he would be interested in any thoughts or experience of anyone having used such antenna for SOTA activations.

Stewart G0LGS


#2

In reply to G0LGS:

Hi Stewart;

There was an article about some experiments on 5Mhz using a dipole resting on traffic cones in Radcom last year, from memory they were quite successful.

73 de Ken G3XQE


#3

In reply to G0LGS:

I have tried very low dipoles with very variable results. If you think about it, this is hardly surprising: a summit with a moist organic soil is going to act as if the earth plane is at ground level as peat and humus is quite acidic and has relatively good conductivity, whilst a rocky summit will have a very low conductivity and the antenna will act as if it is much higher - rocky summits are frequently frost-shattered and the joint system will only hold water at depth. The presence of water has a profound effect.

There have been suggestions that the optimum height for a five megs antenna is about 25 feet, I reckon that this follows because we are too literal about NVIS: a path between say Birmingham and the Lake District is nowhere near vertical incidence!

73

Brian G8ADD


#4

In reply to G0LGS:

Hello Stewart,
I have been doing a bit of research on the net because I have to re-new my 5Mhz antenna quite soon. The place to start looking is military useage. There is quite a lot out there eg:

http://n3oc.dyndns.org/Homebrew%20AS-2259%20GR.pdf

I think it will be a case of experimenting with different designs and see what suits; the old story.

Good luck

Mike G6TUH


#5

In reply to G6TUH:
Hi Mike and all,

A good and informative article. Here in VK a 7 m long “heavy duty” squid pole is a popular SOTA mast. A 40 m dipole can be used in inverted Vee configuration to give good contacts out to 700 km or thereabouts with 5 w on 40 m. 60 m is only available here on an experimenters special licence and not much used.

I have sometimes put the dipole on the top of low bushes (pretty dessicated out here in Oz)and found the dipole still useable thru an ATU.

The basic rule applies. The higher the better. Even if you can hang the dipole from a 20 m + support you will still get a lot of NVI signal. The higher you can go the more efficient the antenna and the better the lower angle (down to 16 degrees) radiation for longer distance QSO’s.

Of course if all you have are walking poles then use them. Just do whatever you can and be prepared for some interesting operating.

73
Ron
AX/VK3AFW


#6

In reply to AX3AFW:

Hello Ron,

Yes I thought the article was pretty useful for SOTA activators - some ideas. My 5Mhz dipole has one end tied to a gutter downpipe and the other end to a laurel bush on the edge of the vegetable patch in the garden. There is a lot of debate about how high,how low, better to have a non connected reflector 5% longer than the dipole below. etc. As I wrote earlier it is very much a question of trying and see!

Bye the way Ron (off topic)I got an email from a USA station today saying that SOTA activations from VK on 7.090 SSB were no good as although they could hear your country they could only operate CW on that band/f. and when they called it was regarded as “CW QRM”. What are the boundaries for SSB activity on the bands for Class A stations? I could look it up I suppose :sunglasses:

2209Z here - bedtime!

73

Mike G6TUH


#7

I’m hoping to play with some Extra Low Profile Antennas (ELPA) when I get a chance.
There’s a lot of info on the web, mostly relating to military usage of on the ground antennas. (OTGA)
This includes the Beverage on ground (BOG) and subterranean antennas.
Probably the best well know OTGA is the commercially produced Eyring ELPA, however it’s a bit big and too complex for most SOTA ops. It does however have a gain of about 4.5dB over a half wave dipole at optimum height (0.2 lambda for NVIS).

Pete
Sorry I couldn’t get any more acronyms in this post!


#8

A couple of US Military ones for sale - new but pricey.

http://www.ebay.co.uk/itm/AS-2259-GR-Antenna-7-Mast-Sections-NEW-/181048740851?pt=LH_DefaultDomain_0&hash=item2a27588bf3

http://n3oc.dyndns.org/Homebrew%20AS-2259%20GR.pdf

Mike G6TUH


#9

In reply to G6TUH:
Hi Mike,
I agree about trying. Any antenna you have is many dB better than the perfect one you are dreaming about.

re off topic:
Haven’t noticed the CW QRM - would be very happy to work cross modes.
In VK the band plan for 40 m is:
7.000 - 7.300 CW
7.030 - 7.040 Digital data modes
7.040 - 7.300 SSB

7.090 is a calling frequency used by SOTA operators who may spread out from 7.080 to 7.120. The US stations would like to see us higher I guess.

73
Ron VK3AFW


#10

In reply to AX3AFW:

Thanks for the information. I think your guess is right. Perhaps a US station or two could chip and say :sunglasses:

Bye for now

73

Mike G6TUH


#11

In reply to G6TUH:
This is undoubtably correct: Generals above 7.175, Extras above 7.125, according to the ARRL chart which I keep in My Documents since the Americans joined the fun!

73

Brian G8ADD


#12

In reply to G6TUH:

A couple of US Military ones for sale - new but pricey.

http://www.ebay.co.uk/itm/AS-2259-GR-Antenna-7-Mast-Sections-NEW-/181048740851?pt=LH_DefaultDomain_0&hash=item2a27588bf3

http://n3oc.dyndns.org/Homebrew%20AS-2259%20GR.pdf

Mike G6TUH

Actually no.
I’m talking ground mounted antennas.
Like the Eyring:
http://www.marsregionone.org/Tech/EPLA_Ant/ELPA-2.html

Four steel tape measures should make a good starting point.
Pete


#13

In reply to G4ISJ:

Hello Pete, tried them, albeit tape measures from ScrewFix - don’t work - unless you want to risk a blow to the finals on your radio!

I tried in various patterns and checked all with my AIM4170. If you can get stainless steel or Beryllium copper tapes without measurement coating then you might be in with a chance.

The reason SteppIR antennas use BC is because it is reluctant to arc.

73

Mike G6TUH

Edit update: If you want this route then you can get a spool of BC from Ron Stone at Vine Antenna (wonderful 110% company & person ) for £40 ish.


#14

In reply to G6TUH:

In reply to G4ISJ:

Hello Pete, tried them, albeit tape measures from ScrewFix - don’t
work - unless you want to risk a blow on your finals on your radio!

Its an interesting article, but it does use the phrase “appears to be…” a couple of times, and I wonder if the EFUs and Splitter are as simple as the author assumes…? I suspect not. But it does encourage experiment with ground level elements!

Adrian
G4AZS


#15

In reply to G0LGS:

After about 100 attempts focused on this problem I came to following simple conclusion in practice on 40 m band and QRP 4W: In terms of the number of QSOs, the diversity of QSOs and the duration of an activation there is no significant difference between the inverted vee shaped dipole with its center at 5 to 7 meters or only at 1.5 meters.

Furthermore, the mountainous ridges are very suitable for OTGA (OTRA, HI, on the rock antenna) operation, the only problem that needs to be addressed is the exact tuning of the antenna.

Yes, when the antenna is up, some reports are better, G, SM, LA stations often give 559 to 579 instead of 339 to 559 for an antenna crawling on the ground, but the vast majority of takers is workable very well.

In my opinion, based on my observation, it is not possible to reach a more accurate conclusion:
If you want to get 579 from G, for 150 meters climb you have to consume about 30 kJ more (tenth of one very light yogurt) and to bring up a pole (2 kg).

mgh*(1/effmw) = 210150*10 = 30 000 J

where
m is carried extra weight
g is gravitational acceleration
h is climb
effmw is estimated overall efficiency of muscular work (only for climbing, still plays a large role horizontal distance)

On the last trip I was a bit hungry, so I was counting calorie consumption :slight_smile:

73!

Karel, OK2BWB


#16

In reply to OK2BWB:
Karel makes a good point about the energy needed to haul the extra weight that a “military style” NVIS antenna (referenced above) would require for a SOTA activation. I like the calculation, Karel! Never thought of it that way. :slight_smile:

Check out http://www.ve3fyn.ca/nvis/
by Warren Paulson VE3FYN. As good a technical explanation on NVIS as Karel’s on calories. I would offer that in my experience in the military the horizontal dipole or inverted vee at 3-4 meters height was an effective and easily constructed antenna for the 80-40-30 m range with solid contacts out to 250-500 km (using the appropriate frequency and time of day considerations to ensure a reliable communications circuit). As Warren points out, an antenna height of 1/8 wavelength for the target frequency is the optimal antenna height for NVIS. That translates to 11 m, 5 m, and 4 m height of your inverted vee on the 80 m, 40 m, and 30m bands, respectively. However, obviously the optimal height on 80 m is impractical for a back pack system on a mountain with few trees. Dropping your 80 m antenna down to 1/16th wavelength height will still accomplish the NVIS task with reduced signal strength.

I generally take a 40 m dipole on SOTA activations and errect it on a 4 m crappie pole in an inverted vee configuration and feed it with light weight ribbon line to an antenna tuner. This works well on 40 m and 30 m NVIS, and can be loaded on 80 m with fair results. I might burn less calories in the process too!
73
Mike, W6AH


#17

In reply to G0LGS:

I have been a member of a Yahoo group for a few years named NVIS.

http://groups.yahoo.com/group/NVIS/

Maybe worth joining.

Sean M0GIA