PL 259 and antenna cable size

[FairweatherDave]

Feeling a bit frustrated with my attempts to set up my AIS and VHF radio this weekend. Plain annoying but that's my problem! Just wish it wasn't such a mission to get to the boat and then fail. Ignoring the splitter and for the time being bypassing it by connecting directly to the VHF or AIS (using a SO239 to BNC) my results have still been very poor. So, following Salty John's helpful tech page I think next stop is to remake the PL259 connection using one of these...
https://www.saltyjohn.com/product/solderless-pl259/
However I am a bit sceptical as I have done a visual inspection of the current one.
My other query is about the antenna cable. My aerial is new (a Vtronics Hawk antenna) which is supplied with 20m of RG58 cable. I have got this running direct from the mast head all the way to the radio without a break, so a long run. However I have read that the longest runs of RG58 should be about 6m. So should I be considering running RG8 from the VHF to the base of the mast (inside the heads compartment) and making a join there? Seems a bit mad to introduce more connections?
Cheers for any replies.

 

[lw395]

I would avoid introducing any joints.
The loss per m of RG58 means you will have about 3.5dB of loss in 20m.
Changing the last few m to RG8 will only save you a fraction of a dB which is easily thrown away by the VSWR of a pair of connectors.
Also a join is a potential water ingress point. Coax should be sealed to prevent oxidation of the conductors.
Proper professional connectors either crimp on or screw together using spanners to compress a gasket.

3.5dB is just over 50% of the signal power, which sounds a lot. But you can only reduce it to about 1 dB using expensive cable.
The good news is that the range gained by having the antenna high up far exceeds the effect of a bit of cable loss.

I've had very good results with the V tronix Hawk antenna (Hawk as in windex?), but if you're not familiar with assembling RF connectors, it's possible to cock it up.
If problems persist, borrowing a VSWR meter might be good.

 

[FairweatherDave]

Thanks. I suspected it was a bad idea. I just hope changing the PL259 will sort it. I have read some of the stuff on the VSWR meters and really it goes over my head. I think next stop would be to try an emergency aerial on the pushpit. I would be tinkering and persisting more stubbornly if it was just popping to a marina but like many it is a whole day out to get to the boat on a swinging mooring and home again.

 

[<152587>]

Before you do anything drastic, the screw-in connector in the base of the mast needs quite a firm push before tightening the thumbscrew. As hawk is DC grounded you will see 10k ohms between the core and the screen if correctly inserted in the base of the mast.

 

[FairweatherDave]

Sorry, bit confused. As you referring to how the cable joins the base of the aerial?

 

[VicMallows]

Sorry, bit confused. As you referring to how the cable joins the base of the aerial?

I am sure that is what 'third engines' meant ....the connection at the antenna.

The 10k ohm resistance check is a very useful test. It will immediately confirm that you have a good connection to the antenna AND that you don't have an accidental short anywhere. (Just in case anyone is checking a VERY old (20yrs +) Hawk antenna, then be aware that early versions show nearly a dead short rather than 10k)

 

[FairweatherDave]

Thanks Vic, having found "third engines" post elsewhere I'm sure that is correct
I also found this
http://www.hawkmarineproducts.com/pdf/V Tronics Instructions.PDF which just about shows it.
Hope I don't need to go up the mast, which seems to be what the 10k ohm resistance check requires. If anyone wanted to spell out to a bit of an electronics numpty how to do that test I am interested!

still hoping it will just be the pl259 connection
I guess the VSWR test would be a lot easier but I don't know anyone with one

 

[lw395]

Thanks Vic, having found "third engines" post elsewhere I'm sure that is correct
I also found this
http://www.hawkmarineproducts.com/pdf/V Tronics Instructions.PDF which just about shows it.
....

Those instructions say the antenna connection is DC grounded with a low impedance not 10kohms.
But I think they are quite old?
You should just measure resistance at the plug on the end of the aerial cable.

 

[FairweatherDave]

Ok. Thanks. So testing the resistance between the core and the screen can be done at the radio end (while I'm changing the PL259).

(The instructions I posted were from a link in a 2016 thread and it was more about the location of what "third engines" was referring to. A rigger fitted my new antenna so I never saw my instructions).
PS Just looked and they are the current instructions on the Hawk V tronics website.

 

[<152587>]

Ok. Thanks. So testing the resistance between the core and the screen can be done at the radio end (while I'm changing the PL259).

(The instructions I posted were from a link in a 2016 thread and it was more about the location of what "third engines" was referring to. A rigger fitted my new antenna so I never saw my instructions).
PS Just looked and they are the current instructions on the Hawk V tronics website.

No need to disconnect the plug, just connect the DVM between the centre terminal and the body. Better if you do it this way as it will identify if there is a possible short in the PL.

 

[VicMallows]

You should do the check AFTER fitting the plug at the radio end of the cable. (Fitting the plug is where any problem is most likely to be created). Just hold one meter probe against the metal plug and touch the other to the center-pin. The polarity of the leads does not matter in this case.

 

[FairweatherDave]

Brilliant. Thanks for spelling it out to me!
Tides are a bit wrong this week for accessing the boat at Emsworth but I might try the kayak from Hayling bridge or the old causeway from Langstone....... desperate to get this annoying thing sorted........

 

[William_H]

You can make yourself a quite efficient VHF antenna useful for emergencies and for testing. You need about 1 metre of RG8 coax and a few metres of RG 58 coax and a PL 259 plug for the RG58. Put the plug on one end of the RG58. Strip the outer skin and braid for 17 inches off the other end of the RG 58 so leaving 17 inches of inner wire plus insulation exposed. This becomes the antenna. Strip the outer off the RG8 and pull off the braid. Discard the rest. Fit the braid over the RG 58 at the antenna end so that the top of the braid can be soldered to the braid of the RG58. The braid from the RG8 is then pulled down for 17 inches. Measure this length again as the braid may stretch. You can get some heat shrink plastic or just tape to cover the added braid. The braid hanging down from the bottom of the antenna (exposed insulated wire sticking out) forms a ground plane or counterpoise. In use you need to hang the whole thing as high as possible on insulating string as high as possible. Hence your choice of length of RG58. You can push it up an old fibreglass fishing rod or tape it to a f/g rod. (not carbon fibre). You will find it works as well as any other antenna depending of course on height. Worth having as an emergency antenna in case of dismasting. Perhaps taped to a wooden boat hook. good luck olewill