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Mobile Antenna (GW7AAV plz Read)


#1

I have posted on here because I think you may have a good idea that others may want to see Steve, hope you don’t mind.
In your images, http://gw7aav.googlepages.com/GW7AAV1.jpg/GW7AAV1-full;crop:0.2,0.27,0.77,0.89.jpg

I think I can see a Diamond SG7900 or the similar. I been looking for an answer to a problem that I have where I need a good antenna for when I am on the move.
I have a 7900 and never considered using it on a rucksack, but next year I have a plan to work some hills in rapid succession and will not have time to even stop.
What I don’t want to do is a handy smash and grab, I need to get a signal out back to base or the station manager (2E0NBR) will not be happy.

Can you give any details on the way you have that antenna set up, and more so what is the ground for it and how is the SWR. Can it be used mounted on a rucksack and used when walking. I know the antenna is a bit of a beast but I also know from mobile car use it’s a good performer.

Thanks
Steve


#2

In reply to 2E0KPO:

Steve, I wouldn’t really recommend that type of antenna as a rucksack antenna, you would do a lot better with just a simple 1/2 wave on a rucksack.

73 Mike


#3

In reply to GW0DSP:

I know what you are saying Mike.

I have used a simple 1/4 wave in the past where I have a chance of hitting things and it was okay but the 7900 may give me the extra performance I will need for the challange planned. The plan is to have the SLAB and main body of the 857 in the rucksack and the control head on the sholder strap in a small case for easy access, same with the mic.

Its a good performing antenna I need to sort out now. I know a handy is not going to do the job and maybe even not a 1/4 wave in this case. (Must keep the Station manager happy. So I am looking at options.

Steve

EDIT, Thanks Mick. I look into that as well


#4

In reply to 2E0KPO:

Apologies Steve I meant 1/2 wave and have amended my original. I found the same results as mick quotes re ground plane and 5/8 wave. I think Steve GW7AAV has abandoned the 7900 as a bad idea himself and was using a collapse-able slim Jim or J pole with far better results.

73 Mike


#5

I have just noticed in another post that Sandpiper do a clip on dipole, I just given him a ring and he has come up with a better idea… a 1/2 wave!! He is going to send all the bits I need to put together a 1/2 wave rucksack antenna. Thats service…

Well Mick and Mike, thanks you were spot on. Thanks GW7AAV for the inspiration!

73 Steve


#6

In reply to 2E0KPO:

I’ve always been very impressed with the customer service at Sandpiper. So you might want to post some photos of what you get before and after it is assembled. I can see a a rush of orders for the same to Sandpiper if it works out well!

Andy
MM0FMF


#7

In reply to 2E0KPO:

Hi Steve,
When I started using the Diamond I had an old aluminium framed rucksack which just happened to be approximately one half wave tall on two metres and around about a quarter wave by just under a quarter wave at the base. This frame seemed to work as a pretty good ground plane and I had no problems with the swr. The antenna was mounted onto another piece of aluminium tube with an L bracket at the top which was fitted with a mobile mount.

I used this set up to successfully keep in touch with my XYL Helen GW7AAU back at home during a number of accents of local hills and it worked very well. It also did the trick when the WX was not too kind and I operated with the rucksack leaning against the trig point a few times.

However I did try mounting the antenna on the SOTA pole by tie wrapping the aluminium half wave pole to it but found that it worked a lot better on the rucksack. The half wave antenna (also a Diamond) that I currently use on the car worked much better on the SOTA pole with the mounting pole basically turning it into a dipole.

I have since replaced my rucksack with a modern internal framed one and have since found the 7900 doesn’t work as well any more (no Ground plane!) so I use the half wave instead (when there are no trees on the ascent). It is not as good but is better than the 7900 without ground planes. For simplicity I have made a roll up J-pole antenna which we Velcro to the SOTA pole beneath the HF antenna on the summits. I did consider that the J-pole could be used as a rucksack antenna on a partially extended SOTA pole.

I have been promising myself for three years that I will make a 2m SOTA beam but still haven’t got around to it. All credit to Richard G3CWI but I hate the idea of those crocodile clips and I am still trying to think of a better way that doesn’t involve a major engineering project or the use of screw drivers on the summits.

Whatever you do best of luck.

Regards Steve GW7AAV


#8

In reply to GW7AAV:

Hi Steve

I had an old aluminium framed rucksack

Still got one! Interested to hear your experiences, I’ll definitely give it a go!

For simplicity I have made a roll up J-pole antenna

I have made one of these, out of 300ohm slotted ribbon feeder. Just feed it over the top of the SOTA pole. Devastatingly effective! (Well compared to the rubber duck it is…)

…2m SOTA beam…I hate the idea of those crocodile clips…

I did too, but took the plunge and have had no problems in two years of hard use. The 3 element version is an excellent aerial, I can recommend it. Can’t get the SWR below 1.7 : 1 whatever I do though…

73 de Paul G4MD


#9

In reply to G4MD:

I have made one of these, out of 300ohm slotted ribbon feeder. Just
feed it over the top of the SOTA pole. Devastatingly effective! (Well
compared to the rubber duck it is…)

J-pole/slim-jim has a fairly low angle of radiation making it an very effective antenna for it’s size particularly mounted high from on the tops of summits. I think one or two people let them flap about in the wind and signals can be very up and down. Investing in a couple of Velcro ties or taping it to the mast can make things somewhat more stable. No replacement for a beam though.

The 3 element version is an excellent aerial, I can
recommend it. Can’t get the SWR below 1.7 : 1 whatever I do though…

1.7:1 is probably acceptable to most people but it sounds a bit high to me and I wouldn’t use it. Is this a common experience or is Paul’s SOTA beam a bit off?

73 Steve


#10

In reply to GW7AAV:

Cheers for the advice

“as for the crocodile clips and still trying to think of a better way”,

I have been using the SOTA beam ever since I started SOTA a few years now and I have only ever had one problem with the crock clips on a summit and I managed to fix it there and then, as I keep a crimp in my spare kit.

To stop them coming off I use small Velcro straps on the coax, from any good DIY shed, this keeps the coax from being pulled on and solves any problems of the clips coming off.

I have to say I never liked the clips either, but I don’t get a problem. I do however hate the small o rings and caps, far too easy to drop and never find again

73 Steve


#11

In reply to 2E0KPO:

I was a bit doubtful of the crock clips too, but I needn’t have been, they have served me well in all wx condx. As for the end caps Steve, discard them and use a dab of red/black paint to distinguish between director and reflector and a turn of red/black tape on the boom ends so as to distinguish the front from the back of the beam.

There is no doubt about it, the sotabeam 3 and 5 are the best designed small yagi for portable use by far.

73 Mike GW0DSP


#12

In reply to GW0DSP:

I must have been the most sceptical person around about the use of croc clips and O rings in the design of the SOTA Beams, but having given an SB5 a thorough testing over the past few months, I can say that I have changed my mind. As with all things, one just has to be careful. A check on the condition of the croc clips should be undertaken before an activation, as with all of the kit being taken out.

I use coloured tape to identify the elements in the following format:

End director - red (“hot” end of the antenna)
Middle director - red and white
Director next to dipole - white
Reflector - black (“cold” end of the antenna)

I also tape the boom next to where the element fits - no brain function required to get the order correct! A similar system is in use on my 6 el 70cms yagi and 5 element modified Tonna.

73, Gerald


#13

In reply to GW7AAV:

1.7:1 is probably acceptable to most people but it sounds a bit high
to me and I wouldn’t use it…is Paul’s SOTA beam a bit off?

I have 2 SOTA beams (always like to have a spare!) and both perform exactly the same so I think it’s inherent in the design.

I’m not entirely happy with an SWR that high myself, but it doesn’t seem to distress my 817 so I live with it for the advantages of the SOTAbeam.

73 de Paul G4MD


#14

In reply to G4MD:

Paul,

As far as I can ascertain from the photos on the SOTA Beams website, the SB3 does not have a coaxial balun in the feeder line. If that is the case, it might be worth trying one as used on the SB5 which has a similar dipole arrangement. Basically it is 8 turns of the RG58 feeder wound over a 70mm length of plastic overflow pipe (I think it is 19mm dia - I have some if you need a piece); two holes are drilled through the pipe at 50mm apart (in line along the length of the pipe). It is formed with the end 100mm of cable projecting from the balun to where the croc clips are fitted. This can be seen from the photo of the SB5.

An SWR of 1.5:1 theoretically represents 4% reflected power, so the SWR you have is certainly less than 10%.

73, Gerald


#15

In reply to G4MD:

I built the MW0IDX design and couldn’t get better than about 2:1 but it seems to work.

However, we should all remember The Gospel of Mathew, chapter 5, verse 11…

“Worship ye not the prophet of SWR for he is a false prophet. Remember ye verily that the SWR of a matched dummy load is 1:1 and it radiateth not one jot.”

:slight_smile:

Andy
MM0FMF


#16

In reply to G4OIG:

Hi Gerald,

You’re correct, the SB3 doesn’t have a balun. I’ve not built one because the aerial seems to work OK without it, and because it would potentially put extra strain on those suspect croc clips - maybe I should try it one day, perhaps the SOTA beam would be even better!

An SWR of 1.7 : 1 represents a reflected power of 7%, which sounds a lot less scary.

73 de Paul G4MD


#17

In reply to G4MD:

it would potentially put extra strain on those suspect croc clips

Yes that’s why Richard recommends using two reuseable ties to fix the cable to the mast before erecting the SB5 antenna. As I generally use horizontal polarisation, I tie wrap the cable to the bracket for vertical polarisation before even presenting the antenna to the mast. It is an easy thing to change it over to the other bracket if I want to go on FM.

An SWR of 1.7 : 1 represents a reflected power of 7%

You can judge the match by the bar chart on the 817. If my HF antenna is too close to metal objects, it shows up as less power going out and I go and tweek it before operating. The SWR monitoring system on the 817 is quite sensitive, so I don’t think you need worry much about the rig, though I seem to recall some people have had their final stage blow. I suppose there is always that risk.

73, Gerald


#18

In reply to G4MD:

I would suggest (unless anyone has a good reason why not) that a balun could be wound around the boom. If it was made of that very thin coax the weight would be negligible and the croc clips would not be under any strain.

All antennas are a compromise and possibly the boom length of the SOTA beam could do with being a bit longer as it is only 1m on the three element which is not much longer than the 2 element HB9CVs I have here, however the proof is in the pudding and the SOTA beam does what it says on the tin and more without being over complicated by fancy matching systems.

Steve


#19

In reply to GW7AAV

I would suggest (unless anyone has a good reason why not) that a balun
could be wound around the boom.

It is my understanding (and I might be completely wrong) that the balun needs to be mounted in the opposite plane to the polarisation of the aerial with which it is associated to avoid the currents that you are trying to eliminate from the outer of the coax being re-introduced, so wrapping it round the boom may be counterproductive.

I have had some success with sleeve baluns made from the outer cover and braid of RG213 fed over the outside of the RG58 feeder, the tricky bit is getting the sleeve the right length and they are obviously frequency sensitive. May try that when I’ve got a spare few minutes. (No time soon then!)

73 de Paul G4MD


#20

In reply to G4MD:

You may well be right Paul but a couple of the commercially constructed beams I have seen have the balun/feed point parallel to the boom. The fact that the booms on these are aluminium would mean wrapping it around the boon would not work but no such problem on the SOTA beam and I see little difference between the two planes of insertion. Maybe someone who knows can confirm or deny your seemingly logical reasoning?

With regard to sleeve baluns - I have tried those in the past and they worked very well and less difficult to make than I had thought they would be.

73 Steve