Cutting coax cable

[oldboot]

Hey Old Boot,

Must admit, I have been around Avionics and antennas in aircraft both on the line and now for many years as an instructor for the past 38 Years and have never heard of what you are talking about.

Please don't take that the wrong way, as I am merely saying I have never heard of it .... you can always learn something new in life, no matter how old and that includes me! Bearing in mind, traditionally I have always worked with antennas with a ground plane readily available.

Cheers
Greg

Greg you have problay been working in a far more civilised and engineered environment. and yes you problay havd a ground plane.....if the plane has anon metal skin I expect they would install a ground plane under the skin.

That is waht the do on things like ambulances with fibreglass bodies.. they install a plate.

These ground independed marine areials are truly weird from a technical pintof view.......I can remember scrathing my head over them when I was a young fella.

In civilised engineering the lenght of the coax is not an issue......and in many fields that is part of the design mind set......"if the lenght of the coax is going to be an issue we will do it another way"

interesting.... regardles of the boat being aluminium or not..... the vast majority of ( small craft )marine areials are ground independent....even when a groundplane is available.

I think part of that is..... that a ground independent areial requires no tuning and any donkey can install one sucessfully with no access to instruments.

If a groundplane antenna was to be installed it would require tuning and an SWR meter or directional wattmeter would be required......

cheers

[Jeremy]

I cut the VHF coax cable on my last two VHF radios to suit the installation. I think the standard cable length must be about 10m, and it cut it back by half or more. My radios worked fine with transmit and receive distances of at least 20 km ship to ship and longer for ship to shore. Yes these were GME radios and GME antenas with plastic bases.
Jeremy

[TheRealAndy]

Jeremy, the problem is not that you cannot recieve or transmit. When you transmit, all power is supposed to be transmitted out the antenna. Detuning an antenna means some of that power is reflected back into your radio, and that can damage your radio.

If you use pacific aerials, then you have no problems as all their VHF antennas are correctly matched.

Oldboot, I snapped one of my VHF aerials in the garage, they are a single piece of stainless wire loaded with a coil at the bottom for matching. Pretty standard design. They are not dipoles, they are a vertical half wave antenna. I had stupidly assumed the the GME antenna was the same design when i said its fine to cut them.

[Fatenhappy]

Hey Jeremy ....

I note you say that you comfortably punch 20 odd k's or more with good results on both transmit and recieve.

Both UHF and VHF (and I'll include 27 meg although it is officially in the HF band) are only ever good for "line of site" transmission. This is prinicipally because of the curvature of the earth.

If your getting 13 nautical miles (about 20 k's) or a little more, that's about all you will get but sometimes in good conditions you could get a little further. This is because the radio wave will actually bend slightly and follow the curvature of the earth a little further than just in a direct line.

Would be interesting to throw a Bird Thru Line on your rig to have a look for what if any reflected power there was. With those results, I would suspect very little, as it sounds to me as though you are travelling quite well.

Cheers

[oldboot]

Saying that my radio goes fine....simply does not cut it so to speak.

With any of these things the only way to know what is going on is to do proper tests either with measuring equipment or by comparison with other "known good equipment.

Most days sitting of wello or green I can clearly hear seaway tower and redcliffe .....moloolaba can be good or fair.

It is hard to assess reception quality as the modulation is FM which tends to mask reception problems till they get quite ordinary or just become unreadable.

This "line of sight" assertion on VHF marine is a bit of an over simplification.....even logging on to raby bay from wello is not line of sight.....

If you are depending on your radio as a safety item you need to have everything going for you.
If the SWR of your aerial is less than ideal, many of the modern radios will reduce output power to protect its self from the reflected eneger from the mismatch.

So instead of having an effective radiated power of 25 Watts....you may have an ERP of 5 or 10 watts..... you will still log on loud and clear with your local VMR..... but if you are cold, wet and in trouble on a week night..... you want to be able to crack the squech at fishermans island where the port and the water police are monitoring.......even more important if you are going off shore.


as for the real andy antenna... so the pacific seems to be a base loaded end fed halfwave.......

My 8" GME has a loading coil part way up the stick........GME arent forthcomming about anything else.

Probaly the easiest antenna to build would be a coax dipole, that would need to be a half wave length could be made to match up, but would need at least a half wave feeder t spec up right.

A colinear would be my preference but it would give a gain more than unity.


any way it is all accedemic..........

Don't cut the coax on your GME and dont crunch up the coax in tight bends either.

A mate of mine was wondering why his 27Mhz radio was naff........the coax was jammed up tight and hard at a right angle comming out of the plug and the exces cable was zippytied up realy tight in a hank with tight bends at the end.

coax should not be bent any tighter than a radius 6 times its diameter......remember you coax is a pipe for radio waves.......it should not be squashed or bent sharply.

coil up you excess in a nice smooth coil and dont pull the cable ties up tight.

cheers

[Fatenhappy]

Can't possibly see how !!! .... "This "line of sight" assertion on VHF marine is a bit of an over simplification" .... because that's exactly how it is ..

Have a look at any reputable publication on Transmission Line or Antenna Propagation Theory and specifically with regards propagation within particular frequency bands of the frequency spectrum.

Whether you do or don't get reception is also made up of many other extenal factors to this statement, including installation techniques, all of which is always an attempt for the best or if not, then the best comprimise.

.... "My 8' GME has a loading coil part way up the stick ........ GME arent forthcomming about anything else."

I have also missed the point here ! From your description, obviously the 8' GME is inductively loaded to compensate for antenna length. So what aren't GME forthcomming about?

Cheers

[Grand_Marlin]

I spoke with Pacific Aerials from NZ at the BLA trade show last year.
In their range of aerials, the VHF can have the coax cut to be shortened or lengthened.
Their 27MhZ can not - the cable forms part of the tuning of the aerial.

As an installer of these items, I don't cut the coax unless there is a genuine need too.

I coil the excess cable in a longer length (say 800mm) then cable tie it.
This way it ties up neatly out of the way.

Cheers

Pete

[foxx510]

If you look at the link I posted earlier in this thread, it also says the cable cutting rule only applies to the GME 27meg antennas.

[Fatenhappy]

Agreed Pete and Foxxy ....

The only problem "could" come in if there were a truck load of cable extra that had been coiled up and even then only on rare occassions via the inductance that is being created by the action of simply coiling the coax.

I would suggest non cutting coax is the exeption rather than the rule and specific to only a few radios, after the added research that I have since done re ground independant type antennas since I started reading this thread.

In any case refer to the OEM for guidance ... too easy!!!

[TheRealAndy]

that makes a lot of sense actually foxx, as to make a matched halfwave 27mhz antenna it would have to be about 5 metres long.

[oldboot]

The line of sight being an over simplification is simply borne out by experience.

200Mhz VHF travels quite well without a direct line of sight......( I spy with my little eye something beginning with A).

I spent many years installing Television Antennae and surveyed many installtions with proper instruments. Analogue VHF television is very close to the marine band and is a much more fussy modulation system and requires (in radio terms) a very very strong low noise signal with very low multipath content.
VHF television travels very well in that "just beyond line of sight"
Brisbane VHF TV from MT Cootha can esily be recieved at Victoria point for example which is certianly not line of sight.
Or the wrong side of Point Lookout ....I did a job over there once....all i could see was a rising hill behind with trees on top......Brisbane VHF yep, Toowomba channel 0...no problem.....any UHF forget it. again not line of sight ( I spy with my little eye something beginning with A)

2 meter amateur radio, is and can be used over considerable distances beyond line of sight.

For many years I drove service vehicles with VHF radios in the 200 to 400 Mhz commercial bands.....they certainly worked beyond line of sight.
As did the old taxi radio system which was VHF, Yellow cabs operated the vast majority of its zones off a mast on the roof of the base at wollongabba which was a far from ideal location. you would not even call Albert st in the city line of sight from Logan Rd at Woolongabba.

I am not claiming that VHF marine will travel anywhere near as well as any of the HF frequencies......and I am not including "irregular aptmospheric advantage".
VHF does work in that "beyond line of sight" or "just over the horison" area... otherwise it would be almost useless..........as any UHF system or land bassed microwave system would be...........

Now microwave is most certainly line of sight........" can you see the dish mate?"...."NUP'..... "forget it then".

It is a simple answer to a common question....... how far will my radio go?......line of sight.............Itis an over simplified answer to a question that is far more complex than the person asking understands.


In recreational marine antennae, "DO NOT CUT THE COAX" is the rule rather than the exception.

The majority of the antennae supplied and used are of a specific type and from a small number of manufacturers... AND they are designed for unqualified people ( the vast majority of marine installers) to install.

cheers

[oldboot]

that makes a lot of sense actually foxx, as to make a matched halfwave 27mhz antenna it would have to be about 5 metres long.

in round terms 18 feet.... yep which is about 5 meters

back in the CB days...... the quarter wave whip all the heros had in the middle their roof was a 9 foot stanless.......Railway electrification sorted that one out .

Also interestin that GME do not publish gain figures (i havnt found them) on their marine aerials.

As to GME not being forthcomming......forthcomming with any detail on their areial.....style of operation, construction, gain, VSWR v frequency plots, radiation polar plots.
The sort of information you would expect to come with a commercial areial or even a TV antenna from a good manufacturer.
Yeh mate its white 8' long and you dont cut the cable.

A ground independent 27Mhz areial less than 18 feet long would have to have a gain lower than unity

cheers

[oldboot]

If you look at the link I posted earlier in this thread, it also says the cable cutting rule only applies to the GME 27meg antennas.

The GME VHF sticks come with the same do not cut instructions.

cheers

[foxx510]

Well get this: I just spoke to GME tech support and they said it doesn't matter if you cut the coax on any of the marine antennas as they are all 50ohm input impedance. So we still really don't know, do we?

[Fatenhappy]

Well get this: I just spoke to GME tech support and they said it doesn't matter if you cut the coax on any of the marine antennas as they are all 50ohm input impedance. So we still really don't know, do we?

Beside any perculiarity of a non grounded type antenna (if you are using one), the input impedance is the be all and end all of this conversation.

It is only when there is a missmatch of the input impedance of the antenna unit that you will achieve any unwanted level of VSWR .....