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Reflector :: Source for RG174 and BALUN

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Post by M0CGH on 16th February 2009 at 14:06
Before I order any coax, I would be grateful if I could sound out on this reflector. I was hoping to get some RG174 coax at yesterday's rally at Osset. I found the rally very disappointing. I took my CW test at the Wakefield rally in 1999, which was held at Thornes Park Stadium, it seems that this rally has gone down hill since then, certainly since moving to the new venue. There seemed to be only 2 vendors selling new stuff and neither of them had any RG174.

I want to feed my dipole with RG174. The dipole will be supported by a fishing pole and the weight of RG58 makes the pole bend too much. I intend to use 80m and 40m mainly, so the loss in the thinner coax is acceptable.

1. Can anyone suggest a suitable source of RG174? I intend to buy about 15 metres. The only place I've ever bought coax before was from Robin Sykes trading as SYCOM. Maplin don't sell RG174 over the counter, only from the internet, and last time I looked it was 99p/metre!

2. I intend to use a Balun to feed the dipole (i want to do it correctly!) can anyone please suggest a suitable one? I've looked at some commercial baluns and they seem really heavy. Weatherproofing seems to be the bit that adds the weight. I'm not keen on 'wind the coax up' choke baluns as these will just add too much weight at the top of the fishing pole and I'm not convinced that they work properly unless they are wound to the correct diameter etc. Can anyone suggest a solution? I'm thinking I've seen a design using a ferrite ring and some windings somewhere. I was nearly serious about dissecting a commercial balun to strip the unneccessary weight, but I thought the cost (around 30) was too much.

73 and thanks in advance.

Colin
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Post by MM0FMF on 16th February 2009 at 14:21
In reply to M0CGH:

Westlake sell RG174 for 50p a metre. I got some from a trader at NARSA in 2006 for 20p a metre. Given the way copper prices have moved 50p probably isn't that bad.

Balun? I have a coax-choke balun but I've never measured its performance. At 6 turns there probably aren't enough turns in it for the LF bands it is used. The weight it presents is certainly not a problem though. Nor would doubling its turns count and thus weight be a problem.

Considering how many people don't use a balun and have absolutely zero problems when out SOTAing I've seriously thought about removing mine. That would however, involve unpacking all the gear when I get home so it tends to stay they way it is because it works so well.

Andy
MM0FMF
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Post by G6SFP on 16th February 2009 at 16:01
In reply to M0CGH:

You can wind the RG174 through an FT114-43 (or similiar size) at the antenna end, as many turns as you can comfortably get on. Then put on whatever connector or connections your going to use for the antenna.

http://www.flickr.com/photos/g6sfp/3284930115/

http://www.flickr.com/photos/g6sfp/3285747654/

Regards,
Nigel. G6SFP.
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Post by F6ENO on 16th February 2009 at 16:16
In reply to M0CGH:

Hi Collin,

I'm using an home made dipole with Collins balun built with RG174 coax.
Never had any problem.
More infos here :http://pagesperso-orange.fr/f6eno/Materiel.htm

Best luck

Alain F6ENO
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Post by G4YTD on 16th February 2009 at 16:22
In reply to M0CGH:
Hi Colin
As stated above, Westlake sells RG174, and more importantly the correct BNC connector to fit to it (so you dont need to butcher a standard connector to make it fit). Unless your support is a monster, I would seriously reconsider the 15m length of cable. RG174 is very light weight cable and in a decent wind resembles a birds nest in no time at all. I have RG174 and mil spec RG58, and use the RG58 most of the time as it is less likely to blow around, and takes a misplaced boot better than the 174. I have used a balun in the past on dipoles, but now only use them at home. I and the people I tested with whilst on SOTA activation could tell no difference between fitted, and bypassed. I now save the weight and dont bother. Purists may have something to say about it, but as a practical kind of person, my choice is not to bother (one less thing to go wrong). The link dipole is worth a look (have a search on the reflector or I can send you info if you are stuck), as it is multiband, and takes seconds to erect/change band. Mine is set up for 10 thru 80m (including 60m/5Mhz) and uses 2mm gold bullet connectors used on model aircraft motors.
Hope this helps.
Tim
G4YTD
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Post by MM0FMF on 16th February 2009 at 16:53
In reply to G4YTD:

The secret to not having the RG174 blow about into a bird's nest and how not to damage it with a misplaced boot is to have a procedure for assembly and dis-assembly of the antenna that you follow rigorously. Finally all that ISO9000 nonsense I've had to suffer for the last 15 years has some payback! ;-)

There's a way of unpacking the bag on arrival, setting up, operating, taking down and repacking that results in no breakages, losses or damage. Follow the procedure to the letter everytime and everything works like clockwork even on a windy summit. The two times I've lost stuff (tent pegs) is because I strayed from the procedure. Whenever I do a joint activation with Brian G4ZRP we have "comments" between us as his procedure is not THE procedure!

15m does sound a little long though. My feeder is 10m which is normally adequate. There have been occasions when this has been too short. Such as Ben Chonzie GM/SS-015 when it was -5C at the summit but well below -15C with wind chill. The feeder was just long enough to reach the stone shelter from the only place to affix the pole. Without the shelter it would have been just too cold to activate. I now have a 5m extension I can add to the feeder so this problem doesn't arise. Normally not needed but it only adds 125g (inc 2x BNC connectors) the total weight.

Andy
MM0FMF
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Post by M0CGH on 16th February 2009 at 18:07
Hi All,

Thanks for the replies so far. I've just had a quick scan through the replies and I thought I'd make the following comments:

1. I know 15m of coax is too long for one antenna, I'm intending to make 2 antennas! My pole is the standard SOTApole, I think it's 7m long including thinnest sections (but I don't use the top 2 sections).

2. I have already made a linked dipole for most bands from 15 to 80m, it is this antenna I'd like to feed with the coax. RG58 is definitely too heavy in my experience.

3. OK on the 'bird's nest'. I think I'm like Andy sounds to be, everything has to be neat and tidy, and packed/unpacked in a certain way or else it upsets me!

4. I have used the antenna without Balun also, I suppose the main reason that people use Baluns is to prevent TVI from the braid of the coax radiating, and not many people seem to watch telly on the summits!

73 and thanks for the replies.

Colin
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Post by G0LGS on 16th February 2009 at 21:36
Hi,

Several months ago I started to make myself a linked dipole using a short length of RG316/U in a similar fashion to that shown by John GW4BVE (http://www.flickr.com/photos/john_clifford/535784209/in/photostream/).

I've not actually finished building it yet (only got as far as tuning it for 10m) but one day I'll finish it off and might then actually do an HF activation or two.

Stewart
G0LGS
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Post by F6ENO on 16th February 2009 at 21:52
In reply to M0CGH:

> 4. I have used the antenna without Balun also, I suppose the main
> reason that people use Baluns is to prevent TVI from the braid of the
> coax radiating, and not many people seem to watch telly on the
> summits!

Balun is used not only for TVI but for radiation patern.

73 Alain
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Post by ON6DSL on 17th February 2009 at 10:35
In reply to M0CGH:

Hi Colin,

PAY ATTENTION and NOTICE: Normal is RG 174 NOT for outdoor use !
It is usually used for projects and intra-equipment wiring.

73
Luc ON6DSL
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Post by M0CGH on 17th February 2009 at 12:49
Alain - yes I know the Balun improves radiation pattern. I understand that a dipole is a balanced antenna and needs a balanced feed to work properly. I was just stating that the most likely reason for people to use a balun at a fixed station is to prevent the feeder radiating close to other radio systems and causing cross-talk. I understand that as a result of the feeder radiating, this affects the pattern from the antenna proper. I have read the information on your web link and found the information very useful, -thanks.

Luc - RG174 is coax like RG58/RG213 etc, the reason that it is used inside equipment is that it is not bulky. As only very short lengths are used the losses are negated also. In any case, I only really operate in dry conditions and only for a short time.

73 Colin


This thread is closed
Post by MM0FMF on 17th February 2009 at 12:51
In reply to ON6DSL:

> RG 174 NOT for outdoor use

I'd better stop using mine then! ;-)

It's done over 112 activations in sun, rain, snow, wind and hail and is still in absolutely excellent condition.

Andy
MM0FMF
This thread is closed
Post by MM0FMF on 17th February 2009 at 12:55
In reply to ON6DSL:

> RG 174 NOT for outdoor use

I'd better stop using mine then! ;-)

It's done over 112 activations in sun, rain, snow, wind and hail and is still in absolutely excellent condition.

Andy
MM0FMF
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Post by G3CWI on 17th February 2009 at 13:08
In reply to MM0FMF:

My 40m dipole is fed with RG174. It must have been out nearly 200 times. Like yours Andy, it is still in great condition.

The external magnetic antenna for my car satnav is fed with RG174 and is always outside on the car - also fine. My GPS timing antenna on the roof of the house is fed with RG174. It has been outside for about 7 years. Ok as well.

If the losses are acceptable, it is a useful cable and, treated properly, is no less rugged than any other.

73

Richard
G3CWI
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Post by F5VGL on 17th February 2009 at 13:39
In reply to M0CGH:

> properly. I was just stating that the most likely reason for people to
> use a balun at a fixed station is to prevent the feeder radiating
> close to other radio systems and causing cross-talk.

I think the situation is opposite now. You use these common mode chokes to reduce QRM coupling through the feed line to your antenna. Radio amateurs use mostly very clean signals, which not always true for other consumer electronics.

73, Jaakko OH7BF/F5VGL

This thread is closed
Post by M0CGH on 17th February 2009 at 14:10
In reply to F5VGL:

Yes, good point Jaakko, I had not thought of that, there does seem to be a lot of stray RF floating about nowadays from all sorts of home electronics!

73 Colin
This thread is closed
Post by F6ENO on 17th February 2009 at 14:23
In reply to M0CGH:
> Alain - yes I know the Balun improves radiation pattern. I understand
> that a dipole is a balanced antenna and needs a balanced feed to work
> properly.

I don't realy know how the radiation pattern is modified without a balun.

May be somebody have information about that and answer the question "is a balun realy necessary for us (on our summits) ?"

Thanks
73 Alain F6ENO
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Post by F5VGL on 17th February 2009 at 16:57
In reply to F6ENO:

Hi Alain,

Just give me the 3D dimensions of your antenna including the feed line. I can check how much EZNEC thinks there is RF in your feed line.

73, Jaakko OH7BF/F5VGL
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Post by F6ENO on 17th February 2009 at 17:49
In reply to F5VGL:

Hi Jaakko

I think it will be rather difficult to give all informations to the simulator because I use 2 kind of antennas show on my website here:
http://pagesperso-orange.fr/f6eno/Materiel.htm

I use the first antenna (Yoyo) without balun.
The second with a Collins coaxial balun.

Each antenna ends by a coil, according to the length of the dipole, i.e. the working freq (either 7MHz, 10MHz or14MHz). I tuned each length in my garden, and it works fine (no ROS) when I'a on a summit.

As usual, I use a fishing pole (abt 5m high), and the dipole is inverted V, each end at abt 1m above ground.

My feed line is a 10m long coax (RG8U).

Best 73
Alain F6ENO


This thread is closed
Post by F5VGL on 17th February 2009 at 20:48
In reply to F6ENO:

Hi Alain,

The 10 m coax seems to be important part of your antenna

http://sral.fi/oh7bf/F6ENOcur.png

http://sral.fi/oh7bf/F6ENOele.png

http://sral.fi/oh7bf/F6ENOaz.png

The symmetry of the dipole is clearly broken due to the feed line. But this is not necessarily bad, since you lower the radiation angle. The fully symmetric dipole is NVIS antenna beaming directly up. There is very large RF current in the coax at 40 m - maybe because the length of the coax is 10 m. The theoretical resonant lengths for the dipole branches are about 9.4 - 10.4 m, which is close to what you are using. The antenna gain is good about 2.9 dBi to 45 degree elevation. I used some average ground model for this calculation.

73, Jaakko OH7BF/F5VGL


This thread is closed
Post by F6ENO on 18th February 2009 at 07:15
In reply to F5VGL:

Dear Jaakko,

Many thanks for your nice work !

When I read G6WRW'spot and antenna description http://www.geocities.com/g6wrw/projects/hfdipole.pdf , I saw that my antenna was working in NVIS mode, not a bad thing for SOTA in EU.

Best 73 and good luck for next "Crt de la Neige" trip...

Alain F6ENO
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Post by M0CGH on 18th February 2009 at 07:21
RG174 coax ordered from SYCOM. 40p/m, 1.75 for correct (small diameter cable) BNCs.

Thanks for replies. Got hold a nice circuit to homebrew a balun too, looking forward to lightweight HF!!

73 Colin
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Post by F5VGL on 18th February 2009 at 08:29
In reply to F6ENO:

Hi Alain,

Of course the model I showed was some kind of approximation to the case without balun or common mode choke on the feed line. With the balun you can expect better dipole symmetry, since it introduces high impedance to the current flowing into the coax.

One more observation from this model was that the antenna gain was reduced by about 3 dB, because the coax was conducting part of the RF current close to the lossy ground. To summarize you should use balun if you want to have ideal NVIS antenna with maximum antenna gain straight up. Otherwise it is not so important.

73, Jaakko OH7BF/F5VGL
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Post by F6ENO on 18th February 2009 at 08:39
In reply to F5VGL:
HI Jaakko,

Do you use "4nec" or "Mmana" soft for the simulation ?
I find that both are not very easy to use... hi !

73 Alain
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Post by F5VGL on 18th February 2009 at 10:22
In reply to F6ENO:

Hi Alain,

EZNEC by W7EL

http://www.eznec.com/

73, Jaakko OH7BF/F5VGL
This thread is closed

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