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Mini 8 / RG8 coax


#1

I know that there is a school of thought that a piece of wet string will do for a feeder on a summit, but I do not subscribe to that opinion. Having actived some shielded summits (GW/MW’s, etc) has reinforced my view that the choice of coaxial cables is an important issue, especially for the higher frequency bands.

Currently I am using RG-58U for 2m (as supplied with the SOTABeam SB5) and a 4.7 metre length of 5D-FB 7mm semi-rigid for 70cms, both with BNC connectors. I now need to sort out a decent feeder for 23cms and I am tempted to obtain another length of 5D-FB, but this is rarer than rocking horse droppings. I have only see it available as a made up lead for mobile use and of late only short lengths have been advertised.

While searching for the source of a suitable feeder, I came across Mini 8 cable which I understand to be equivalent to RG-8 / RG-8X which has better characteristics than RG-58U. I wonder whether anyone is using this cable on the higher frequencies and if so whether it performs reasonably, both electrically and physically. Suggestions for an alternative would also be appreciated.

73, Gerald


#2

In reply to G4OIG:

There’s a coax loss calculator at http://www.ocarc.ca/coax.htm which has a good selection of cables to pick from. Enter the length and frequency and see the loss. If you don’t want to work out how the loss affects your power yourself, enter the power and you’ll get the power at the antenna.

In my case for 10m RG174 on 5.4MHz with a measured 3.9W out of the 817 I should get about 3.4W at the dipole. I can live with that kind of loss. My 2m J-pole is fed with 5m of RG174 and I get 5W on 145MHz so I’d see 3.35W at the antenna. Not so good but I can live with that as many contacts are LOS.

In your case Gerald LMR200 looks like it might be a go-er. Assuming that for 23cms you are prepared for a bit less cable length and to sacrifice a bit of flexibility, 3m of LMR200 at 1296 fed by 5W would give 3.8W at the antenna. Not bad for a flexible cable the same size as RG58 on 23cms. You can buy LMR200 in the UK for a pound a metre from Solwise ( http://www.solwise.co.uk ). There maybe other dealers with better prices. Google is your friend!

Going up to 3m of LMR400 which is RG8 size you’d get 4.5W for 5W in. Thicker and heavier of course. Also available readily in the UK.

I don’t know how difficult it is to fit connectors to either of these cables or if you need special types.

Andy
MM0FMF


#3

In reply to MM0FMF:

Hi Andy,

Many thanks for the advice. I hadn’t thought about looking at the WiFi market for a solution. I should have done though as I was recently reading about a 2.4GHz link between two houses in an urban situation - 6 miles apart!

The 400 cable looks fine. Weight is not really an issue if I can get the losses down and I’d like a bit more lee-way on the length. The idea of using Mini 8 was that I would use that on 70cms and use the 5D-FB on 23cms, but this is a much better proposition.

I very much appreciate your help… err, you don’t have a few 23cms watts to spare do you?

73, Gerald

Postscript at 23:00. Just done some calcs. Think I might try using the 200 cable first - not a lot of cost for a short length plus a couple of SMA’s. Using the 400 cable would mean N types which are significant weight add-ons. Could be overkill. The downside is fiddling with SMA’s in near zero degrees with horizontal driving sleet…


#4

Gerald,

My solution to 23cms on the summits (I have never got around to trying it yet) was going to be to mount my 24 element beam on a photographers (mini) tripod and to tape my IC12E to the 8 inches of boom behind the reflector. I would only need 3 or 4 inches of coax (which could be thick as you like) to make the connection then so I could get almost my full 1 watt into the antenna. If necessary a longer mike lead could be fitted for ease of operation.
So far I have only had contacts using the rubber duck or with the beam hand held. However if I take my serious camera gear(rather than the compact) the tripod is already in the bag so the only extra weight is in the beam/coax.

Regards Steve GW7AAV

PS I have always thought that manufactures should look at mast mounted radios and then you only have to run a control/power cable instead of all that lossy feeder.


#5

In reply to G4OIG:

Morning, Gerald.

At the higher frequencies, you are of course right. Loss is important - as much on recieve where it adds directly to the system NF.

For what it’s worth, here is a little table. Assuming 4m cable with a VSWR of 1.25:1 gives the following (using my own calculator which uses curve-fitted data from the manufacurers quoted figures as it’s input):

5D-FB 1.01dB 320g
RG58 4.53dB 100g
RG400 2.88dB 256g
FSJ4-50B 0.59dB 840g
LMR-200 1.61dB 120g
Quickform 141 1.77dB 143g

BTW the figures for RG58 are very much dependant on the manufacturer of the cable at this frequency. Your 5D-FB looks pretty competitive.

GL,
Richard


#6

In reply to G4OIG:

I very much appreciate your help… err, you don’t have a few 23cms watts to
spare do you?

Funny you should say that. When I was looking about for something to use as a small PA for 70cms I found that I’d bought a MM 144/1296 transverter, put it away and forgotten about it. I think we should be able to arrange some interesting activity weekends later in the year for 70cms and 23cms. So whilst I have some watts (well one and a bit) they aren’t spare.

Andy
MM0FMF


#7

In reply to G4ERP:

Richard, do you know if ordinary N and BNC conectors can be fitted to cable like LMR200/LMR400 or do you have to use the special crimp connectors that are available for this cable?

Andy
MM0FMF


#8

In reply to G4ERP, MM0FMF and GW7AAV:

Hi Richard, Andy and Steve,

Many thanks for the information and comments. It seems that I will need to revise my ideas yet again. I am considering putting a linear / preamp close to the antenna with just the handheld at the operating position, but the difficulty is how to switch the relays from a handheld running only 280mW. It needs real discipline to switch the arrangement separately and I’d be sure to transmit up the back end of the preamp while under attack from the elements.

The connectors advertised on the Solwise site are crimp types and are not expensive. The issue is crimping them correctly - it is daft to have to buy a tool just for one or two connectors. I never seem to get it right with the tools that I have and prefer to solder my connections… maybe I am just a duff engineer… no, not an engineer at all!

It seems that there is a reasonable level of interest in the higher bands. I will be QRV on 23cms from North Wales in February or March and I am planning to repeat my trip up to GM later in the year, so hopefully suitable arrangements can be made to try for a few S2S contacts on the band.

73, Gerald

Postscript at 12:45. After further thought, I’ve decided to bite the bullet and order up a length of HDF400 (LMR-400) and a couple of clamp fitting N types (yes, Solwise actually do these - missed them on my first look at their site). I priced up the 5D-FB and with the special BNC’s or N types from Nevada, the cost was getting up to around £30 for a 3.6m feeder. As Richard says it is good cable, but it hard to source. I can easily tell the difference between it and RG-58 on 2m on a 4.7m length. LMR-400 should be even better… having thought about it, I am prepared to carry the extra weight. CU on 23.


#9

In reply to MM0FMF:

Hi, Andy.

I haven’t used these cables and looking at the data sheet, there isn’t enough info to deduce whether standard connectors would be okay. The ones shown look remarkably similar to Tyco/Greenpar, though. I’d be tempted to get hold of a sample of the cable and see.

At 23cm, it’s got to be correct so if I wasn’t sure I’d leave well alone - or test it on the analyser.

Richard


#10

In reply to G4ERP and MM0FMF:

Hi Richard and Andy,

I’ve done the deed and ordered up 4 metres of the cable and a couple of the clamp N types. I’ll do a comparison to the standard N type as far as I am able, but the price of the dedicated connectors at Solwise is very reasonable - much better than I’ve seen standard N types from some other suppliers. If either of you want to see the cable in real life, a few inches off the end won’t hurt. I’d be pleased to send you a sample.

73, Gerald

Postscript at 17:00. Must say I’m impressed by Solwise. Only a small order for them, but it is being despatched today 1st Recorded and the invoice in .pdf format came by email.


#11

Hi again Richard and Andy,

The LMR-400 arrived today. The 4 metre length weighs 466g. The special clamp N types look very similar to standard ones, but have centre pins to suit the larger diameter centre conductor and the threaded sleeve at the back of the plug has a slightly larger diameter bore since the cable is 11mm semi-rigid. The connectors weigh about 45g each, so 550g or so for the whole assembly. Loss estimated as 0.64dB and so the cable will provide a better performance than the 5D-FB and not that far off FSJ4-50. Now all I need to see is how easy it proves to use in practice on a summit!

73, Gerald