Which coax

[Graham_Wright]

There is a difference between the SWR readings from my two masthead aerials.

If the coax is at fault, what do the forumites recommend for replacement?

 

[matthewriches]

I assume we are talking about VHF? How long is your run? RG58 is standard 50 ohm gear. There are many variations of this but ensure you get BRAIDED inner.

I tend to use LMR400 where I can and this is what my HAM setup uses but otherwise good RG58.

 

[matthewriches]

Also,if pulling a new masthead coax, pull 2... 1 as a spare.

 

[Graham_Wright]

I assume we are talking about VHF? How long is your run? RG58 is standard 50 ohm gear. There are many variations of this but ensure you get BRAIDED inner.

I tend to use LMR400 where I can and this is what my HAM setup uses but otherwise good RG58.

My mast is 42 feet, the total to the radio around 50. There seems to be quite a range available with price inversely proportional to losses.

 

[Salty John]

A common recommendation is to use a cable that loses no more than 50% power in the run from radio to antenna. This is 3db. If you race under World Sail regulations, you can lose no more than 40%, so 2.2 db.
RG58 (approx. 5.5mm dia.) loses about 6 .2 db per 100 feet. So, around 35 feet run is OK.
RG8X (6.5mm) loses about 3.75 db per 100 feet. So, about 60 feet.
After that you’re looking at RG213, a 9.5 mm cable.
The above are generalisations – there is a bit of variation from brand to brand.
Centre conductor should be stranded, not solid, to avoid metal fatigue, and tinned copper is better than plain copper to avoid corrosion. The braid coverage is important, over 90% is good. Braid plus a foil barrier is also good.
There are several variations on the RG cable types.
Coax sizes are often given as 6mm, 7mm and 10mm, referring to the diameter. Very few are exactly this, depends on the thickness of the pvc jacket.
Hope this helps.
Warning! I sell cable so there is a commercial element to this post.

 

[matthewriches]

My mast is 42 feet, the total to the radio around 50. There seems to be quite a range available with price inversely proportional to losses.

Some good quality RG58 would be ok and with all new connectors and no joints, you'll be good to go.

LMR is good but you'll need more room and probably a special tool as they are generally crimped.

 

[john_morris_uk]

The price of the cable is often related to how much copper is in it. Tinned braid with a good coverage is not cheap. This is one area where I make no attempt to skimp.

PS. At VHF enough of a length of waterlogged corroded cable can give a very reasonable VSWR reading regardless of whether there’s an antenna on the end or not.

 

[Graham_Wright]

Warning! I sell cable so there is a commercial element to this post.

I know and the two antennas were supplied by you! I'll send you a pm to avoid embarrassment!

I have brought the two antennas below and used a 1 metre patch cable to the SWR meter. The results are confusing.

 

[Dockhead]

A common recommendation is to use a cable that loses no more than 50% power in the run from radio to antenna. This is 3db. If you race under World Sail regulations, you can lose no more than 40%, so 2.2 db.
RG58 (approx. 5.5mm dia.) loses about 6 .2 db per 100 feet. So, around 35 feet run is OK.
RG8X (6.5mm) loses about 3.75 db per 100 feet. So, about 60 feet.
After that you’re looking at RG213, a 9.5 mm cable.
The above are generalisations – there is a bit of variation from brand to brand.
Centre conductor should be stranded, not solid, to avoid metal fatigue, and tinned copper is better than plain copper to avoid corrosion. The braid coverage is important, over 90% is good. Braid plus a foil barrier is also good.
There are several variations on the RG cable types.
Coax sizes are often given as 6mm, 7mm and 10mm, referring to the diameter. Very few are exactly this, depends on the thickness of the pvc jacket.
Hope this helps.
Warning! I sell cable so there is a commercial element to this post.

The voice of a professional.

Why not RG214 instead of RG213? I'm using 214 because of the double shield and silver coated conductor. It's more expensive. If you have room in the mast for a fatter cable, I think this is a good way to go. I wouldn't use LMR400 for marine VHF (although I use it for ham VHF/UHF) because of the foam dielectric, which makes the cable more vulnerable to being compromised by being pinched or kinked, and probably more vulnerable to water (although there are bury-grade versions of LMR400).

And one shouldn't get hung up on the last dB of attenuation due to the coax spec. Marine VHF is 25 watts which is massive overkill. The really important thing is a really good installation with really good and properly installed connectors, and the minimum number of them. I use a Type N connector at the top of the mast and a continuous run of coax all the way to the radio with no connector at the mast base.

With good installation and good and well installed connectors, RG58 is perfectly adequate.

With my installation (adding a good antenna to the mix, a Shakespeare Galaxy ungrounded internal dipole), I get decent signal reports from across the Channel -- on 1 watt of power. My VHF is normally set to 1 watt.

So you can certainly afford to lose half the power in the coax provided you are not losing even more in badly installed or cheap connectors.

 

[lw395]

The voice of a professional.

Why not RG214 instead of RG213? I'm using 214 because of the double shield and silver coated conductor. It's more expensive. If you have room in the mast for a fatter cable, I think this is a good way to go. I wouldn't use LMR400 for marine VHF (although I use it for ham VHF/UHF) because of the foam dielectric, which makes the cable more vulnerable to being compromised by being pinched or kinked, and probably more vulnerable to water (although there are bury-grade versions of LMR400).

And one shouldn't get hung up on the last dB of attenuation due to the coax spec. Marine VHF is 25 watts which is massive overkill. The really important thing is a really good installation with really good and properly installed connectors, and the minimum number of them. I use a Type N connector at the top of the mast and a continuous run of coax all the way to the radio with no connector at the mast base.

With good installation and good and well installed connectors, RG58 is perfectly adequate.

With my installation (adding a good antenna to the mix, a Shakespeare Galaxy ungrounded internal dipole), I get decent signal reports from across the Channel -- on 1 watt of power. My VHF is normally set to 1 watt.

So you can certainly afford to lose half the power in the coax provided you are not losing even more in badly installed or cheap connectors.

I'd agree with that, but some people are bound by the Offshore rules which implies keeping loss down.
Valid comment about kinking foam dielectric cable, you can reduce the risk by stiffening with heat shrink at strategic points.
Fatter cable is more weight aloft, plus usually a bigger bend radius. No point installing a nice big cable then kinking it around to reach the aerial.
If you are serious about antenna performance, keeping the damp out is paramount. A little oxide on the braid can up the loss hugely. Quality connectors are a good start, and they must fit the cable properly.

 

[William_H]

There is a difference between the SWR readings from my two masthead aerials.

If the coax is at fault, what do the forumites recommend for replacement?

Should I guess that OP has a ketch and so 2 masts. It would be undesirable to have 2 VHF antenna in close proximity on one mast top. As said a vswr reading is a measure of power reflected back to the transmitter because the antenna is not matched to the transmitter and cable. Thus a perfect cable or VSR meter at the base of the antenna will give a true measure of antenna tune and operation. If however you measure the vswr from the transmitter end then the losses of the cable both up and back again will reduce the power being measured as reflected hence giving a better vswr reading. A fault in the cable ie open or short if at the top will give a poor vswr reading which is masked by the losses of the cable. A fault at the bottom of the cable will give a dramatically bad vswr reading. ie reflected power is not attenuated by the cable run.
Hence a difference of VSWR reading of 2 antenna with different length cables might give a different vswr reading even though both working correctly.
Indeed a very long cable with losses will give a perfect VSWR reading even with no antenna connected.
So to a large degree actual apparent range can be as useful in checking antenna performance. Bearing in mind that apparent strength diminishes in a anti logarithmic manner. ie you hardly notice great losses in antenna or cable until eventually no signal at extreme range. VHF radios are frequency modulated such that the receiver has circuits to reduce strong signals and enhance weak ones so that strong and weak sound mostly very similar.
All the info given on cable types and losses are all good. ol'will

 

[Graham_Wright]

I have a lot to digest from all this! Both aerials are mounted on one mast. They live at the extremities of a one metre long beam mounted athwartships. Both cables are interrupted at inboard connectors primarily to permit (a probably useless) defence against lightning.

I need to understand what is going on when I connect the antennas via a short patch lead below decks. There seems to be no reading on the meter. Presumably there is little reflected signal from such a short lead but the output indication should still show. The meter is a crossed pointer version.

I am fearful of blowing the radio transmitter output. The set default to high power when changing some channels.

 

[lw395]

I have a lot to digest from all this! Both aerials are mounted on one mast. They live at the extremities of a one metre long beam mounted athwartships. Both cables are interrupted at inboard connectors primarily to permit (a probably useless) defence against lightning.

I need to understand what is going on when I connect the antennas via a short patch lead below decks. There seems to be no reading on the meter. Presumably there is little reflected signal from such a short lead but the output indication should still show. The meter is a crossed pointer version.

I am fearful of blowing the radio transmitter output. The set default to high power when changing some channels.

Sounds like either the meter is broken, the transmitter is broken, or the transmitter has detected a poor VSWR and shut down?
Try a different lead from transmitter to meter.
You should get a near 100% reflection from a short patch lead.

 

[Graham_Wright]

Sounds like either the meter is broken, the transmitter is broken, or the transmitter has detected a poor VSWR and shut down?
Try a different lead from transmitter to meter.
You should get a near 100% reflection from a short patch lead.

I have three meters - all (roughly) the same.
The transmitter transmits (to nearby receiver).
Why full reflection from a short lead?

 

[lw395]

To transmit to a 'nearby' receiver might only need less than a milliwatt, depending on what you mean by nearby.
That could be leakage straight through a blown power amp stage.
Or a shut-down PA stage switched off due to bad VSWR sensed.

The signal which goes into a short cable will not be much attenuated by the cable, the open end of it will radiate a little, most goes back to the transmtter as a reflection.
You don't need long cables for the signals to be modelled as a wave, the maths is the same for loads connected directly to sources.
An open or a short puts all the power back into the source, just at a different phase.
Either can destroy the PA, or PAs can be designed to tolerate or manage bad VSWR.

Personally, I'd test the transmitter directly into a suitable RF power meter.
Or you could try it with a known good antenna. Irritate someone with radio checks and see if the 12V current drawn looks convincing?