Masthead antenna

S

sailorbaz

Well-Known Member
Joined
22 Apr 2002
Messages
296
Location
Bald Knob, Queensland, Australia
Visit site
While my mast is down, I plan to replace the VHF whip aerial on the rear quarter with a masthead antenna which takes a PL259 plug. The local chandlery sells RG213U co-ax but I remember seeing on here RG58 being recommended.
Which is best?
Also how do I make a connection and a waterproof fitting to go through the deck so that the mast can be removed in future? It's a deck stepped mast.
Thanks for any advice
Sailorbaz
 
sailorbaz said:
While my mast is down, I plan to replace the VHF whip aerial on the rear quarter with a masthead antenna which takes a PL259 plug. The local chandlery sells RG213U co-ax but I remember seeing on here RG58 being recommended.
Which is best?
Also how do I make a connection and a waterproof fitting to go through the deck so that the mast can be removed in future? It's a deck stepped mast.
Thanks for any advice
Sailorbaz
Click to expand...

Check with the radio manufacturer about the impedance it is expected to work with. I'd go for the best spec VHF cable for the job. Salty John (seen in these waters at times) can provide good advice on the matter (just a satisfied customer...)
Does your mast have a cable channel inside? Many do. It stops cables rattling and fouling halyards. Otherwise be prepared for some minor hassle.
I originally had a supposedly waterproof PL259 through deck connector. Not! . I replaced it with a conventional cable gland , taking the VHF cable below the coach roof before making a connection there. Nice & dry, but covered with amalgamating tape anyway. I don't need to take the mast down when ashore so it won't need disturbing much.
Feeding VHF cable through a mast is I think worse than running a halyard for which you have no mousing line..! Serious patience, assorted lengths of fence wire and tiny fingers help!

Graeme
 
RG213 ( and RG8U) is 10mm cable and therefore difficult to work with, but loses only 2.5 dB per 100'.
RG58 is 6mm nominal diameter and extremely lossy, about 5.5 dB per 100'.
RG8X is 7mm nominal diameter and around 4dB per 100' loss.
A 3dB loss halves the signal strength.
Use RG58 for runs of no more than 6m, RG8X up to 25m and RG213/RG8U for longer runs.
All are 50 ohm, not 75 ohm like TV coax.
Make sure your cable is marine quality - tinned core and braid, stranded core.
 
I think it is safe to say all VHF communications radios work with a 50 ohm antenna and so 50 ohm cable. RG 58 is the common and cheap choice.RG213/U is a better quality cable but much more expensive. The only other type of coaxial cable used is 75 ohm for television reception. So quite common. Beware also some cables might look like coax but are screened cable for audio use.
Coaxial 50 ohm cable comes in all sorts of quality. Usually this equates to loss. Looking at RG58C/U from one supplier has a loss of 17.4 decibles at 100 megahertz for 100metres of cable. RG213U is quoted as 5.9 decibels per 100m at 100megahertz. Now 17db might be serious loss for a long cable considering marine VHF is more like 160 megahertz. 20 decibels of loss will see only 10% of your transmitter power getting to the antenna. In practice for small boats with mast say less than 10 metres long RG 58 is OK. This is about 6mm in diameter. RG8/RG213CU cable on the other hand has vastly less loss so is more desirable for bigger boats and longer able runs. This is the 10mm aprox diameter cable. Much more expensive and heavier.
You will get better range out of the radio with mast top antenna. Actual apparent loss of stength is logarythmic so a 3watt hadheld may not sound much weaker than a 25watt fixed radio considering smaller low antenna etc. But at extremes of range you want high power and high antenna.

A little explanation here. The characteristic impedance of a cable is given in ohms as is the impedance of the antenna and (even loud speakers). This is vastly different to the ohms you might measure with your multimeter. ie the resistance of a light bulb or heating element.
When you have power to transmit you have a choice of high voltage low current or low voltage high current. Any cable used to transmit electricity has losses. The high voltage can tend to leak across the insulation while the high current tends to be lost in the series resistance of the conductors. So any pair of wires or cable will have a ratio of voltage to resistancee where losses are lowest. High voltage low current cable (the spark plug leads on your car) will have a high resistance. Any high current will be lost but high voltage is OK. Called High resitance (impedance) cable On the other hand the cable to your starter mortor carries high current. Lots of copper but insulation is not great. So a high voltage on that cable might be lost but it can carry high current without loss. (low imnpedance cable). Now at radio frequencies the power or signal can be lost by the capcitance between the conductors. (inner and outer) and by the inductance and resistance of the conductors. Because this loss is quite high at radio frequencies we need to choose a cable design to best suit our needs of carrying power from transmitter to antenna. The transmitter ratio of voltage to current can be described as a resistance. We call it impedance and likewise the antenna will best convert voltage and current to radio waves at a preferred ratio. The transmitter cna be made to match the antenna then we need a connecting cable to join the 2 which will carry that power with minimum loss.

Have a look at power transmission cables for 50 hertz power. The power being transmitted is huge. It has been found that for cables of 100 miles or more the cheapest cable for highest power with lowest loss comes at about 500 thousand volts at perhaps 1000 amps. A ballance of cost of big insulators so voltage doesn't jump over but high enough voltage so curreent is lower so reducing cost of the wire itself. Yes that is 500 megawatts about a big citie's use. Fortunately with the use of transformers we can choose what voltage and current we want to transmit. ie match the power house impedance to the transmission line.

For a cable of perhaps half a metre at VHF the impedance is not critical because the losses are small. But the longer the cable the more critical the impedance match.

So assuming you go RG 58 you will need to buy a plug or adapter to suit the samller 6mm diameter. I suggest that before you onnect to the antenna you put your nmultimeter across from centre to outer and check DC resistance. This is sometimes a short circuit and sometimes open depending on the antenna design. If in future you doubt the natenna cable you can check from the trnasmitter plug and you should still get the same reading. If itr is a short circuit now then later a high resistance will indicate a broken connection. If it is a high resistance now then later a low resistance will indicate a short in the cable.

You can get through deck connectors but as said I think the best is a swan neck type bent tube to allow the cable and connector to go throught he deck while excluding waterwhere any connectors apart from the one to the radio can be inside. So less connectors is less trouble. In any case keep your existing low level antenna as a standby or for AIS. good luck olewill (my appologies for the crude mathmatics)
 
William_H

Don't you have RG8X cable down under? Excellent compromise in performance and convenience (as mentioned also by Salty John).
 
Thanks for the replies. I've just been to measure the cable run and the mast is 8.5 m plus 4.5 m from the base of the mast to the radio, which makes 13 m or about 43 ft.
We don't seem to have RG8x in Oz but I can order from the US on eBay. A 50 ft roll should do it.
I'm hoping to find a deck gland where the hole is big enough to pass a PL259 plug and the gland then seals on the cable. That way when I take the mast down I can just disconnect it.
The guy whose boat is next to mine in the yard, has to cut the plug off each time he drops the mast - so the cable gets a bit shorter each time!
Sailorbaz
 
For the join below decks you may use a BNC connector, much smaller than a PL259. Make sure you source 50 ohm variety, though. I use crimp on types, though others prefer soldered ones, I'm not convinced that it makes a difference in a protected environment, but would be more concerned above deck. Having said that, many antena manufacturers use press-on fitting cables at the masthead.

Rob.
 
You must log in or register to reply here.

Share this page

Top