VHF Cable-tech question.

Seagreen

Seagreen

Well-Known Member
Joined
30 Jul 2005
Messages
2,299
Location
Tied up away from the storm. Oh yes.
Visit site
I have found some 17m of VHF Aerial cable in my garage. Lucky me.

However, is it suitable for the new-ish DSC VHF radio I'm about to install?

The cable is 5mm in diameter with the code markings:

RG-58C/U-MIL-C-17/28A

Is this suitable for a Standard Horizon VHF with an AIS engine and signal splitter? I would just PM the guy I bought it off, but knowledge shared is wisdom gained, etc.
 
RG-58C/U-MIL-C-17/28A
***************************************

Depends if it is 75 ohm or 50 ohm

If 75 ohm then keep it for your ethernet or tv coas thats what you bought it for....if it is 50 ohm then it is OK even for DSC /forums/images/graemlins/smile.gif
 
RG58 is, IIRC, 50 ohm so it is OK for your VHF. Can' t say if it is low loss however, which you may want for the very best performance. Look on Maplin's site for specs.
 
So how would I tell whether its 75 ohm or 50 ohm?
It was attached to an old VHF aerial, so maybe that's a big clue...

Doesn't the cable markings give it away? I could always just suck it and see, but I don't want to have to de-rig the mast to insert a new cable if it isn't suitable.
 
RG58 is 50 ohm.
It's really for short runs although you see it in up to 18m lengths supplied with some aerials. In these longer runs it can lead to loss in performance. RG8X is better for runs up to about 60', RG8U or RG213 for longer runs.
Not all coax is created equal but probably difficult to tell the quality without knowing the manufacturers spec for the cable you have.
Braid coverage is important, some coax has a lowly 80%, better stuff is 96% or more. The dielectric, the bit between the centre core and the braid, should be solid, not foam, to prevent wicking up moisture.
I wouldn't use RG58 of any quality to run to the masthead; others will tell you it will be fine.
 
Here is a co-ax cable Loss Calculator .

RG213 is low loss, but is both expensive, and about the same diameter as a garden hose. RG58 on the other hand has much higher losses, but is thin and cheap (particularly if you have just found some!). If you want a compromise, look at marine grade RG8X.

John
 
You rarely really notice the losses in marine VHF because there should be plenty of power to reach the horizon. But it has to be said that RG58 is really crap and occasionally that will make the difference between communicating and not.
 
Looked at the other way - all VHF installations require the same cable - what you have will suit the installation electrically (yes - rg58 is 50 ohm).

Running that sort of cable to the masthead is likely to result in a 2-3dB attenuation of the signal (which might not sound much - but is in fact half the power of the transmitter /forums/images/graemlins/frown.gif ) However I imagine almost all marine installations do use that sort of cable - anything with lower loss is going to be a lot fatter and stiffer and so much harder to run to the top of the mast.
 
[ QUOTE ]
You rarely really notice the losses in marine VHF because there should be plenty of power to reach the horizon. But it has to be said that RG58 is really crap and occasionally that will make the difference between communicating and not.

[/ QUOTE ]I might not have phrased it in exactly that way - but it sums up the situation fairly well!!
 
According to the loss calculator, the RG 58 loses about 40% over 32' which is what I think the run will be, and the RG8X seems to lose about 30%. The aerial height is going to be on a mizzen about 4m above sea level so would this drop in efficiency make any practical difference?
 
If you are talking about the difference between 40% and 30% drop, no that is too small to be significant in practice - it equates to a theoretical reduction in range of the VHF of less than 10%. In practice when transmitting on high power the range of the VHF is likely to be limited by "line of sight" rather than power constraints.
 
I think the short answer is that your RG58 cable will work. Many VHF aerials are sold with RG58 fitted, and you don't hear any complaints about cable losses.

In my case I replaced all the mast wiring last winter while the mast was down. The old co-ax was in a pretty bad way, with black corrosion extending about 25cm into the cable. It was probably about 20 years old. This was probably due to poor quality cable - RG58 is pretty universal, and large quantities have been made, much of it for interior use. I decided to spend a bit more on fully tinned, marine grade RG8X cable, in the hope that it would last for the life of the boat. The deciding factors was cable quality rather than losses.

John
 
The new antenna I installed this year came with RG58 cable. When I tested it I was able to talk to the CG using an aerial 8M from my position, on 1W power (with a pretty large metal obstruction - the Forth Bridge - in the line of sight if that makes a difference). I know only slightly more than next to 0 about the technicalities but that test suggests to me that my antenna is transmitting a reasonable signal so I wouldn't be too worried about it.
 
In Marine radio only line of sight is used and a loss of 3 dB makes no difference. You got plenty of power. RG58 is ok, but there is better cable available

In amateur radio it is a different story they fight for every dB to keep the loss to a minimum.
Unfortunately these radio hams don’t understand that marine radio suits a different purpose.

(I am a radio ham as well DL1EEU ex G5ERB/ G4YYZ and a professional engineer)
 
You can talk to the CG with just about anything - they have fantastic equipment and very high antenna locations. It's one of the reasons that calling the coastguard for a radio check is not recommended - better to call for a radio check from other boats and then ask the location of the responder - if they don't volunteer it in their response.
Talking to another boat in rough conditions is probably the worst case and it is in worst case scenarios that you need every little bit of the system working in your favour.
 
When I am in a bad mood, I let replies like that really annoy me.

There have only been two answers to this question:

1. Those who have said I called x on RG58 so it must be OK.
2. Those who said its not optimum but it will make little if any practical difference.

Group 1 people did not really understand the issues. Group 2 people did and gave practical, consistent, sensible, informative, accurate answers. Who could ask for more ?

So comments like "Unfortunately these radio hams don’t understand that marine radio suits a different purpose." and "(I am a radio ham as well DL1EEU ex G5ERB/ G4YYZ and a professional engineer) " really piss me off.

(Lucky I am in a good mood!)
 
So how would I tell whether its 75 ohm or 50 ohm?
********************************************

If you google RG 58 you ll see that some makers sell RG 58 as 75 ohm as well as 50 ohm ???????

Well if you got is as connected to a marine VHF then it probably is 50 ohm....maybe it says on it...
 
Re: RG58 & RG59

The sister to RG58 - 50 ohm cable is RG59 - 75ohm cable. 75 is usually used for TV antennae and in most cases 50ohm would work just as well but as with the comparison for VHF, it is not correct nor optimum. There are other factors involved including the size of the connectors as the diameter of RG58 is a bit less than RG59, but again this is often "overlooked" but may well work OK in the circumstances.
 
You must log in or register to reply here.

Share this page

Top