Testing the vhf cable to see if the aerial is good working condition

[Beyondhelp]

It's possible to get a 'good' reading when the system is bad. For instance, if the braid has corroded. This is because the meter measures the ratio between the signal going up and the signal reflecting back - if the signal going up is low due to a corroded braid, the ratio can be erroneously in the 'good' range.
The SWR meter is no substitute for checking for bad connections and corroded cable. If those issues are OK, then the SWR meter will tell you if the antenna is OK or not.

That's not what I have experienced.
RF does not behave like normal electrical current that most (almost all) people are used too. You'll get reflections at any impedance mismatch, which could be a loose plug, broken coax shorted connection etc.

This usually show up quite easily. If you cut the shielding/screen outer you will still get a bad SWR (try it, if you wish - I wouldn't recommend it however, as all this reflected RF energy is sent back to the output stage and can damage it).

What is important is you calibrate the FWD power in order to correctly measure the reflected power.

I consider anything over 1.5 to be unacceptable in the VHF range, others may say otherwise.

I did mine recently as this pic above just about shows. At 12v I had a FWD power of 25 watts, and REF of about 0.5 which is fine.

Also, I have used CB devices when I've had nothing else to hand, and they have so far all worked, BUT the power readings are widely off, but this doesn't matter since all your interested in is the ratio between FWD and REF power, which is a SWR of about 1.3 in my case.

In fact you can calculate various SWR conditions here:

http://allenk.home.infionline.net/vswr.html

Quality and type of antenna coax is it should also be pointed out (if its not obvious) very important. RG58 coax is ok for short runs of a few meters, but I'd highly recommend to use RG213 for anything more. Not only is its loss half as much as normal RG58 it also protects devices near it better from the RF energy causing a nuisance of itself.

One last thing WHY do VHF manufactures still insist on using cheap low quality RF connectors. (SO239 and PL259) I just don't get why they don't use proven N-type connectors. (Or perhaps they do on really expensive marine stuff?)

.... HTH

 

[Salty John]

I think it was clear from his original question that the OP is inexperienced in radio matters - a caution that the SWR meter is not a panacea seemed appropriate. False readings do occur. A bad reading means you still have to go up the mast to detach the antenna to find out if you have a cable short or an antenna gone bad.

Regarding cable size - I agree that RG58 is inappropriate for cable runs of more than a metre or so. It's unfortunate that it's often offered, as a package with an aerial, in lengths of up to 60' (18m)! More than half the power is lost in the cable. On the other hand RG213 is very difficult to handle, particularly for amateurs, because it is a 9 -10mm cable with very tough insulation. Difficult to make connections, difficult to run in confined spaces such as through a mast or under a headlining. For long runs, over 60', I'd suggest RG8U which is the same diameter but with a more user freindly construction. Losses are the same as for RG213, about 2dB per 100' at 100MHz.

Superyachts aside, for most boating applications I like RG8X, a 7mm cable, with attenuation of 3.5dB per 100', as apposed to 5dB for RG58. It's an easy cable to handle and make connections with. Of course, all marine cable should have a stranded core and tinned copper core and braid. RG8X meets International Offshore racing requirements for signal loss up to about 60'.

On the question of connectors, I can see why radio manufacturers stay with the SO239 socket. It's simple and robust and it allows the consumer the relatively easy task of fitting a PL259 as opposed the more complex, or fiddly, N-male. On some big base antennas you can specify N connectors but you would probably be asked to order a minimum quantity as it's non-standard. Also, some bigger antennas will come with a short factory attached RG213 cable with an N connector on it to attach to the ships cable run. The N connector is a good in-line connector. Personally I don't object to two PL259's and a barrel connector, but concede it is a large and clunky connection.

Lots of diverse opinions on radio matters (a bit like anchors!) - the forgoing has been mine!

 

[Beyondhelp]

Nice reply! I agree with all of that except my hatred for the sockets!

 

[LittleSister]

Very useful info on Marine VHF installation, troubleshooting, and on fitting PL259 connectors (I've just found I've been doing it wrong all these years!) in the 'Articles and Links' section of Salty John's website.

 

[rob2]

I've always though that the PL259 was cumbersome and more suited to the old, bulky commercial equipment than a small yacht. I used a converter on the back of the fixed set and a bnc on the cable so that I could hook up my handheld to the masthead aerial should the fixed set fail.

A few years back the battery pack on my antiquated handheld faded out, so I bought an Icom M31 and its aerial seems to be on an N connector! Let's hope I never want to connect it to the masthead aerial, then.

Rob.

 

[Beyondhelp]

N-type connectors are too large for handheld. It cant be that on the Icom?

 

[CharlesSwallow]

That's not what I have experienced.
RF does not behave like normal electrical current that most (almost all) people are used too. You'll get reflections at any impedance mismatch, which could be a loose plug, broken coax shorted connection etc.

This usually show up quite easily. If you cut the shielding/screen outer you will still get a bad SWR (try it, if you wish - I wouldn't recommend it however, as all this reflected RF energy is sent back to the output stage and can damage it).

What is important is you calibrate the FWD power in order to correctly measure the reflected power.

I consider anything over 1.5 to be unacceptable in the VHF range, others may say otherwise.

I did mine recently as this pic above just about shows. At 12v I had a FWD power of 25 watts, and REF of about 0.5 which is fine.

Also, I have used CB devices when I've had nothing else to hand, and they have so far all worked, BUT the power readings are widely off, but this doesn't matter since all your interested in is the ratio between FWD and REF power, which is a SWR of about 1.3 in my case.

In fact you can calculate various SWR conditions here:

http://allenk.home.infionline.net/vswr.html

Quality and type of antenna coax is it should also be pointed out (if its not obvious) very important. RG58 coax is ok for short runs of a few meters, but I'd highly recommend to use RG213 for anything more. Not only is its loss half as much as normal RG58 it also protects devices near it better from the RF energy causing a nuisance of itself.

One last thing WHY do VHF manufactures still insist on using cheap low quality RF connectors. (SO239 and PL259) I just don't get why they don't use proven N-type connectors. (Or perhaps they do on really expensive marine stuff?)

.... HTH

Well, I went up to about 1:1.7 but I accept all you say. Since the introduction of PLL transmitters around 15-20 years ago, the expected level of VSWR should be lower; as you say 1:1.5 should be considered a norm these days and 1:1.1 is perfectly achievable.

Chas

 

[William_H]

PLL transmitter

What is a PLL transmitter? In my mind it is a Phase Locked Loop the modern way to generate multiple channels with just one crystal. Compared to old days when each channel had a transmit and receive crystal.
I can't see how this change would improve Antenna VSWR except that the PLL allows for more channels so wide band range so in fact poorer VSWR at frequencies beyond the middle of the design frequency.
Or are you confused with the VSWR sensing in modern transmitters which throttles back the power to avoid damage when VSWR is bad. (as in no antenna)
I have been out of the industry for a long time so maybe acronyms have changed. Please explain. olewill

 

[Lady Campanula]

'My brain 'urts!' reading all this...

It seems one has to buy/have expensive 'use once then never again' kit to make and keep a VHF transceiver serviceable, where a masthead fit is involved.

There's a mix of cable types, another mix of connectors, a VSWR test box, then there's soldering kit and knowhow.... Even the right kind of crimpers can cost as much as the VHF set!

Certainly half the boats I've sailed on over the years carried VHF failures 'cos the owner couldn't fix it, didn't want to climb up the mast to wiggle at bits he didn't understand, and was loath to employ a marina tech costing as much as the kit!

What's wrong, I ask myself, with a pushpit-mounted antenna and a robust cable, robustly connected all the way to the VHF set, that simply works - and stays working?

​

 

[Barry Jones]

Nothing - you just don't get the range on a pushpit mounted antenna that is only a few metres above water.

IMHO VHF's is one of those things that gets made into a bigger issue than it is.

If the VHF is installed properly from the start, then it will give years of faithful service.

The problem is that the install is often botched and then problems start to manifest very quickly.

'My brain 'urts!' reading all this...
What's wrong, I ask myself, with a pushpit-mounted antenna and a robust cable, robustly connected all the way to the VHF set, that simply works - and stays working?

​

 

[CharlesSwallow]

What is a PLL transmitter? In my mind it is a Phase Locked Loop the modern way to generate multiple channels with just one crystal. Compared to old days when each channel had a transmit and receive crystal.
I can't see how this change would improve Antenna VSWR except that the PLL allows for more channels so wide band range so in fact poorer VSWR at frequencies beyond the middle of the design frequency.
Or are you confused with the VSWR sensing in modern transmitters which throttles back the power to avoid damage when VSWR is bad. (as in no antenna)
I have been out of the industry for a long time so maybe acronyms have changed. Please explain. olewill

To put it simply, because the introduction of that generation of transmitters allowed for the transmitted frequency to be manually and critically tuned without relying upon the etching of a crystal to obtain optimum performance which stayed in tune for around a month at best, right?

God! There's always some bloke on here who has been a resident of Coton House and been a victim of Arnie Weinstock's "turn the masses into electronics nurds" programme of the 60s & 70s.

Chas

 

[Lady Campanula]

Nothing - you just don't get the range on a pushpit mounted antenna that is only a few metres above water.

Well, I'm fairly conversant with quasi-optical range and anomalous propagation. Also that the line-of sight can be calculated using the following equation:

D = 1.33(SQRT(2Hr) + SQRT(2Ht))
Where
D = distance to radio horizon (miles)
Hr = height of RX antenna (feet)
Ht = height of TX antenna (feet)

I'm not going to argue over constants 1.4, 1.2 or 1.33... 'different ships, different long splices', but ran the numbers through my little calculator to see what theoretical difference it might make.

Using K=1.4 and assuming a shore antenna height of 144 feet ( simplicity ) and, first, an antenna 'mast' height of 32 feet, then an antenna 'pushpit' height of 8 feet, my machine suggests :

Masthead ~ 24.72nm
Pulpit ~ 20.76nm

.......or a difference of about 4nm range.

Choosing a higher shore antenna of, say, 256 feet ( simplicity ) give a similar notional range difference.

Frankly, I'm not all that worried about losing 4nm notional range, if I can have far higher Tx signal strength and system reliability.

I'm tempted to believe that a decent antenna fitted on a pushpit arch, where inspection and maintenance is easy, and any failure is swift to fix, might serve me better that a masthead unit with a longer cable run ( more attenuation losses, more corrosion ) and the need to break and remake a good cable join every time the mast comes down/goes back up again.

I don't like mast-climbing, and I expect to use VHF for comms with stations within my visible horizon. Should I have a serious emergency, I have a 406MHz EPIRB.

What's missing in my musing?

 

[Beyondhelp]

To put it simply, because the introduction of that generation of transmitters allowed for the transmitted frequency to be manually and critically tuned without relying upon the etching of a crystal to obtain optimum performance which stayed in tune for around a month at best, right?

God! There's always some bloke on here who has been a resident of Coton House and been a victim of Arnie Weinstock's "turn the masses into electronics nurds" programme of the 60s & 70s.

Chas

That makes no sense at all. Most antennas have a bandwidth of at least a few Mhz. A VCO VHF or non PLL Set would be entirely useless long before the problem of SWR due to lack of PLL could even be remotely considered.

I'm a bit bemused by that comparison... (My experience is 10yrs of FM broadcast PLL exciters, amplifiers and Antenna design...) Although out of that industry now still engineer for a few local FM RSL community setups...

 

[William_H]

To put it simply, because the introduction of that generation of transmitters allowed for the transmitted frequency to be manually and critically tuned without relying upon the etching of a crystal to obtain optimum performance which stayed in tune for around a month at best, right?

God! There's always some bloke on here who has been a resident of Coton House and been a victim of Arnie Weinstock's "turn the masses into electronics nurds" programme of the 60s & 70s.

Chas

What is this all about. I know I am a foreigner but I am really confused. Crystals are very reliable for really long time. I have one here still spot on from WW2. And anyway all VHF sets rely on the accuracy and reliability of one crystal. olewill with BS antenna twitching

 

[Barry Jones]

Nothing, unless you want to speak to another boat rather than a coast station or coastguard. whats the range to speak to another boat with a 2m antenna height ?

As I said, people tend to make a lot of fuss about little.

I don't like mast-climbing, and I expect to use VHF for comms with stations within my visible horizon. Should I have a serious emergency, I have a 406MHz EPIRB.

What's missing in my musing?