Ahh a man of science.......
cheers
Ahh a man of science.......
cheers
You supply the aerial and I'll supply the SWR (ta RealAndyOriginally Posted by foxx510
) and the cutters.
I intend on living for-ever....so far so good
I don't even have a radio in my boat, let alone an antenna to test..... The cutters I have.
got an old 27Mhz radio and aerial to play with and a roll of coax.......come to think about it I have a 30Mhz swr power meter somewhere.
sounds like a reasnoable excuse for some cold cans and a couple oF KG of prawns.
cheers
To realy stir the thing up.......the instructions also recomend only using pre made extender cables supplied by GME.........now is this marketing or is there a specific feeder length.
I'm definitely with Oldboot on this one and meaning nothing more than what I am asking, "I also wonder if this done to only boost the sales of GME" What's wrong with an identically made other brand of cable ??
Here's a thought boys and girls ....
I'm going to do some rewiring tomorrow to F&H (weather being OK).
I'll just change things around and also do the rewire for the installations of the VHF and 27 Meg systems as well.
Both have a stack of coax that I am going to cut to suit the application. So I'll crank them up and do a comparison before and after the chop with a bird thru line SWR meter that I can borrow from work.
I am refitting Navman and yes I know they are not the GME's in question but it should give an indication of system change in any case.
You are questioning my judgement........OK, I am prepared to meet up with someone with a 6' GME antenna and UNCUT cable at a set point and lets do the numbers with another station somewhere else.
Jeremy
"The underlying spirit of angling is that the skill of the angler is pitted against the instinct and strength of the fish and the latter is entitled to an even chance for it's life."
(Quotation from the rules of the Tuna Club Avalon, Santa Catalina, U.S.A.)
Apathy is the enemy
As I mentioned before......any "judgement" that does not involve proper instruments is going to be poor when frequency modulation is used.
You can not make the same qualitative judgments of signal strenght and clarity with FM as you would with AM by listening.
Consider all of these modern radios will have automatic gain controll on the recievers
I note also that the majority of VHF marine radios do not have a signal strenght meter.
The only time it will be possible to make listening reception judgments will be when nearing the effective range.
To make realistic assessment we need to measure.
SWR or return loss. to establish how much power is being lost by reflection.
It may then be helpfull to measure field strenght at a given distance.
cheers
Next time your cruizin' past drop in and we'll bung the SWR on and see what the go is
We'll just have to use that as an excuse to have a cuppa and chinwag.
How's the munchkin going?? Driving the boat yet??
All of this mumbo-gumbo is purely theoretical and based on best case scenarios and in reality a radio/aerial installation on a boat is usually a fair bit less then ideal when you take into account the environment they are working in and the manner in which they are sometimes installed.
So how many people have actually have had a SWR on their radio sets when they're new let alone when they're 12 months or 24 months or 36 months old.
Personally I do not think the radio/aerial combination is not worth crap if it is not tested regularly to see that the installation is still working effectively.
I intend on living for-ever....so far so good
Personally I do not think the radio/aerial combination is not worth crap if it is not tested regularly to see that the installation is still working effectively.
Interseting point finga when you consider the crappy salt water marine environment that all our gear is normally subjected to.
I'm as guilty as most and have to admit most of the time don't go near the radios for any servicing, just give them an eyeball now and again and in particular I am one who should no better !!!
Cheers
Greg
Hi Guys,
I asked this question back in 2006, also sent an email to GME, this is the response I received to this dilema and an explanation of why......some interesting findings covered in page 3 of the link.
Link to thread - http://www.ausfish.com.au/vforum/sho...highlight=coax
Well after all the good info that was recieved I thought I would also ask the manufacturers some questions. So I wrote to GME and explained the situation.
Also when I posted the original question and started this thread I also posted it on Fishnet, on there I recieved several mixed responces. Any way today I recieved a reply from GME, it is not very tech compared to some of your responces, someone may wish to escalate the issue to the GME Engineering Dept in Sydney as the author has suggested.
FC.
The theory of antennas, transmission lines (cable) and radio wave propagation, is possibly the most complex and least understood of radio related subjects.
In addition, it should not be coiled up.
Coiling the cable forms inductive loops (like a transformer) and makes the cable prone to spurious electrical activity (spark noise from the electrics of the motor) and other sources.
Lay out the cable as best you can. Zig Zag it across and back, under the cabin.
This specific length of coax forms a vital part of the whole antenna circuit, it assists in a number of important functions such as, impedance matching, loading the transceivers input and output circuits and acting as a ground plane, i.e. earth.
Extra lengths of coax can be added if necessary, but again the extra length is very specific. Contact GME.
The AE87 is a Ground Independent Antenna and the coax cable attached to it, must not be shortened. It’s designed and manufactured that way.
The connector is left off simply to assist the installer, that’s all. Nothing special here.
If the PL259 type connector was pre-fitted, a large hole would need to be drilled to accommodate feeding the cable through the gunwale (gunnel) or dash.
Antenna VSWR measurement, in this situation, is not recommended. Again it’s a very complex issue.
Measuring continuity with a Multimeter can also prove misleading as there is some circuitry in the antenna base.
Take care when fitting the connector. It is easy to short the inner conductor to the shield or connector body.
Fibreglass hulls and metal hulls require different antenna types. One has a ground plane (tinnie) the other fibreglass doesn’t.
The Internet has many sites covering the differences between Ground Independent and Non Ground Independent antennas.
This is a brief answer, I hope it helps.
For further information contact the Engineering Dept at GME Sydney.
Regards
Warren Bates
QLD Service Technician
I am no techo guru like some of you guys appear to be, but as a USER of VHF, all I am concerned about is transmitting and receiving radio messages. I have found that my radio with cut aerial coax cable gives the same signal strength and clarity of reception numbers of 5 and 5 with other ship and shore stations from the same distannce as other boats in my vicinity, THAT IS ALL THAT CONCERNS ME.
The radio is my previous boat with cut coax cable outperformed other boats in my vicinity for various reasons. ie I was able to talk to other boats and hear conversations which they were not. I don't know what they were whether battery, age and type of radio, age and type of aerial, quality of the electrical connections, age and quality of the mike. Whatever.
It works at least comparably to VHF radios in most other boats, and that satisfies me.
Jeremy
"The underlying spirit of angling is that the skill of the angler is pitted against the instinct and strength of the fish and the latter is entitled to an even chance for it's life."
(Quotation from the rules of the Tuna Club Avalon, Santa Catalina, U.S.A.)
Apathy is the enemy
Yeh there is a lot of hokus pokus and heresay floating arround here.
I cant say that I am impressed with either the concern, the technical understanding or the information given to the GME staff.....they seem to be very poorly informed on a variety of matters considering they are suposed to be radio coms specilists.
All the TV antenna manufactures I used to deal with were all to keen to explain the technology in the areials and why it was better than their competitors....... and they expected their staff to know too.... what a contrast.
Even the storeman at some of thes places could give you the technical spiel.
It is sad that no body seems to value good technicians anymore.
I have discussed this with my mate Dan who I have known for over 20 years... he was an apprentice 6 months ahead of me and has over 20 years of specilaist radio com experience and been arround boats for much of that time in his spare time.
There are definitely issues with the areials mentioned, there are definitely reasons why the coax is supplied a specific length.... the issues on 27 MHz and VHF marine are similar but slightly different.
the coaxes on both areial groups will be multiples of a quarter wave lenght.....( there may be a bit of technical fiddle length too)
All the concepts of tuning, bandwidth and "Q" become much less critical ( generaly) as frequencies go up.
The chances of cutting a coax on a VHF aerial and "lucking out" on a good cut are very much better than on 27Mhz.
With these coax matching sections (and transmission lines generaly) it is the first quarter or half wave length is far more critical then the following lengths.
so if I have a 27Mhz antenna with say 9 feet of coax.... that represents one quarter wave length....if I chop that anywhere it is not likley to spec up well.
If however I have a VHF stick with say 6 meters of coax the quarter wave length being 1 meter (not real figures), I should be able to cut that in 4 or maybe 5 places and get a good match......if I cut a very short piece say 25mm of the very end... like I would to reterminate a dodgy plug the liklyhood of a bad effect is very low.
If I was to get the chop 25mm wrong near the first quarter or half wave point the effect is more likly to be bad.
Depending on the design of the areial (this we do not know) the chopping effects could be less or greater.
If the length of coax is a " minimum length stub" ( not tuned but a minimum lenght) chopping the coax past the first half wave point is likly to have very little effect ( in my hypothetical at 2 meters or longer.)
I think our chances of getting a proper description of the design and working of one of these areials is pretty slim.
As for a Chopping a VHF coax and getting it right......I don't think the odds are bad........
Now as for coiling up coax.........this claim of inductance that would effect performance............you have to coil it up pretty small and quite a few turns to get a significant effect.......I recon if you were to coil the coax any bigger tha 6 inch coils you would have difficulty measuring anything in the coil.
It was and is a very common practice in commercial radio ( even up to microwave frequencies to place two coils of coax below the antenna in installtions to allow for retermination......and or to allow for flexibility where the feeder enters the aerial.
Back when I was young and spotty, I played with a few ground independent designs.....some of these had some sort of coax coil at the base to provide loading or to force a quarter wave point in the coax.........these coils had to be very tight to be effective and or have a ferrite core.
coiling up in nice big coils should be no problem.
As far as losses in coax....RG58 ( depending on brand and spec) should have about 10db loss per 100m at 150Mhz.....so the losses in less than 10 meters are not even worth calculating....saying that it is better to have a continuous length of 10 meters of coax rather than two x 5 meter lengths is definitely true.....typicaly... in radio coms a connector join is rule of thumbed at 1db.....so there is possibly more los in a connector join than in 10 meters of coax.
just for interest the VHF marine band is between 156 and 162Mhz... which sits just below VHF TV channel 6.....
Oh concering those push together plugs supplied with the areials.......i took one look at one and decided it was not suitable for a marine environment.
I didn't even like it in a clean and dry envronment.......there was no way you could seal it up.
No thanks crimp or solder
cheers
Surely it only matters on the specific GME antennas which don't have a 50 ohm input impedance though? Or are we saying now that maybe none of them have a 50ohm input impedance?
Whoa, hold up for a tick their. I am not saying you dont know anything. But both you and oldboot are firing back and forth with generalised statements which are not entirely correct. It may be that you are just simplifying things, but if you are then perhaps state it so I dont show ignorance. For now, I will bow out of this topic cause its way off track and I seem to be offending people (which I am really good at BTWLost me therealandy .....
Dunno why I and others need to be told ...... "but I would love to extend on the topic of rf propogation as it is one topic I do know a fair bit about. (suggesting I and others perhaps don't) .....
..... and then, .... "But may I suggest both of you go do some research on propogation, and just remember that VHF is a broad term, from 30 to 300 MHz. Also just remember that you may still be able to see a target, and not get reception (research Augustin-Jean Fresnel). One more thing, as the frequency gets higher, it eventually becomes light. Think about light and materials it will and wont pass through ..... "
Two points here ....
1. So why do I need to think about this ? ... in any case whos talking about Xrays, Gamma Rays, Micro waves and the like ..... where's all this supposed to be going? ..... and,
2. Hmmmm, I didn't realise that "we" know nothing or even "less" persay than you therealandy since we need to do research on the topic ... interesting point of view. Particularly since Antenna Propogation is one of my bread and butter core subjects that I instruct on regularly !!!
No I don't know every thing about the subject, but I do know a fair amount and nor would I be dicatorial in my ideas, but rather throw them toward the forum for general consideration !!!
Going back to my previous recommendations to Mad-One, .... all should be OK, but always consult the OEM if in doubt with regards cutting the coax.
Cheers and have a good one .....
For the record, I do also have extensive experience in radio comms. I have done commercial design of both antenna's and rf equipment including radio's. As an apprentice I did a lot of work on remote telemtry systems, so I also have plenty of field experience, not just academic knowledge. I was also a amateur radio operator (no longer hold a licence) and did all my certificates when I was only 15, so I have been around RF for a few years now.
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