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Where is VHF Actually Used, Please ?


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Ah, Mike.

You've discovered my pet hate.

 

I absolutely abhor VHF's, chart plotters, GPS, echo sounders and any other piece of navigational equipment that beeps.

 

I believe the designers do it so it gives positive confirmation of a pressed button.

 

Drives me crazy.

 

A lot of equipment will allow you to switch it off in the settings. Have you looked in the handbook ?

 

If it can't be switched off, I don't buy it..

 

Why does it beep so much and what does it mean? Do all VHF radios do this? Is there a way to switch off the beeping apart from increasing squelch?

 

Edited to add - I had a look in the Silva S12 manual and it doesn't mention it. Might be worth giving their office up in Scotland a ring.

Sometimes, you can switch it off by resetting the memory or holding a certain key sequence down - they don't always put these things in the manual.

Edited by NB Willawaw
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I use an amateur radio 2-meter band aerial with about an inch and a half cut off the end.

Allan,

 

I've been trying to modify a 5/8 wave 2-metre antenna, (which is what I think yours is) for use on Marine VHF, but am struggling with your "cut off an inch and a half".

 

I can't immediately see where I'm going wrong.

 

Say I have a 2-metre cut correctly for the 145.500 MHz FM calling frequency, that I wish to convert to Marine.

 

Seems sensible to optimise around channel 14 or 16, being ones you might actually end up using, (as most you will not on the cut). Lets choose 16 at 156.800 MHz.

 

 

So 145.500 MHZ on Amateur has a wavelength of 2062 mm, and 5/8 wave is 1289 mm.

 

On the other hand 156.800 MHz on VHF Marine has a wavelength of 1913 mm , and 5/8 wave is 1196 mm.

 

To me that implies reducing a 5/8 wave antenna by something around 1289 - 1196 mm, namely 93 mm.

 

I make that about 3.66 inches, so am coming up with more than double your number - clearly a bit more than rounding errors!.

 

I suspect I've gone wrong somewhere, but can't see where. On the other hand as the marine frequency is pushing 8% higher than the amateur one, without thinking too deeply, it does sound like the length reduction should be of that order.

 

Care to educate me ?

 

WHOOPS...... Forgot the 95% included in Julian's comments when actually considering an antenna, rather than just the actual wavelength.....

 

So, (trying again!)....

 

So 145.500 MHZ on Amateur has a wavelength of 2062 mm, and 5/8 wave is 1289 mm. That's (95%) 1225 for the actual whip.

 

On the other hand 156.800 MHz on VHF Marine has a wavelength of 1913 mm , and 5/8 wave is 1196 mm. That's 1136 for the actual whip.

 

To me that implies reducing a 5/8 wave antenna by something around 1225 - 1136 mm, namely 89 mm.

 

I still make that about 3.5 inches, not 1.5 inches......

 

Help, somebody, (please!)

Edited by alan_fincher
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What I do, is use theory to get to the right ball park, then re-construct the whip out of solid copper wire and about 15% too long ( I have some 50 mtrs on a coil of approx 14 gauge ) and start snipping off whilst TX'ing at the desired centre frequency, until lowest SWR is seen....

 

I do this for a 1/4 wave, as this can be soldered onto a banana plug or even just wedged into a SO-239 on the roof with a matchstick - For 156 Mhz, I would start at approx 19" as that's about a 1/4 wave whip's length on 2M (145 MHz)

 

The 5/8's aerial is a 1/2 wave, with the first 1/8th of a wavelength wound into a coil to help match the aerial to 50 ohms. So the length of the whip is going to be approx 2X the length of a 1/4 wave, and the coil is of approx length of 9.5", coiled into 3 or 4 turns at the bottom and pulled out to give wide spacing - this is likely to be in stainless steel anyway and not alterable - that's fine - it will be compensated for adequately by the length trimming.

 

Remember to stand a bit away from the aerial when you transmit, and don't have any metal objects ( chairs etc) on the roof near the antenna, as this will alter the apparent length needed a bit.

 

The easy way is to make up the aerial a bit long, and stick it onto a MFJ-259, sitting on the boat roof, dial up the centre frequency and trim to lowest SWR - always best to trim in the actual final location of the aerial if possible, for obvious reasons.

 

My favourite aerial is a Slim Jim or J-pole, which for Marine VHF will be about 4 feet long, ground plane independent and about 6dB better than a 1/4 wave, or equivalent in going from 6 watts to 24 watts TX power ( Hand-held to fixed set). This is because it's effectively a matched, and properly fed, folded dipole where the angle of best take off of the signal is a very low angle - typically around less than 10 degrees, compared with a 1/4 wave of about 45 deg, and a 5/8ths of about 30 degrees, thereby putting the max energy out where you want it - almost horizontally. I use this type of aerial on the bike, stuck to the rear of the screen, and on 70 cms ( 433 Mhz) approx 12" tall for the radiating part and 6" for the matching stub - range with 4 watts is around 8 miles bike to bike - more in open country / flat terrain, which is more than fine for our needs.

 

Google for J-Pole and Slim Jim for design ideas, which give formulae for making suitable aerials for any frequency between about 50 MHz and 500 Mhz.

 

Height is good for range - a higher gain aerial also helps but will need to be maintained more accurately vertical ( or in the case of Yagis, more accurately pointing at the transmitter). Having a good coax will help especially if the feeder length is long - Get and use proper 50 ohm coax, soldered connections, and the minimum of tight bends, don't crush the feeder, and try not to use adaptors - get the correct connectors, Silver (or Gold) plated with PTFE insulation within - every little short cut taken will take its toll and the result will be "why did I bother" as opposed to a big smile. Use self amalgamating to seal up outside connection liable to get wet and cover SA tape with pvc tape to stop sunlight destroying it. I have found Evo-stick glue to be great for sealing things up - it seems environmentally inert and lasts for years.

 

If you want a small, directional aerial, the HB9CV is a two beam yagi, Gamma-matched "Yagi", and provides excellent results for its small size - again you can google for plans to make for any VHF / UHF frequency. I have one for each of the 145 and 433 MHz bands and they do a great job in identifying the direction of the transmitter, without being too critical to hold the direction accurately.

 

Amateur shops which have a good range of bits and pieces of gear include...

 

Martin Lynch and Sons - Chertsey

Waters and Stanton - Hockley, Essex

Nevada - Portsmouth,

Radioworld - Birmingham

Rocket Radio - Letchworth.... and several more - all with good websites and helpful guys at the end of the phone line.

 

Hope that helps some people - Amateur radio helps get boating people communicating ! :lol:

 

Nick

Edited by Nickhlx
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I have a Silva S12 handheld - which I don't find easy to operate. Last time I used it on a night trip from Limehouse to Teddington I couldn't hear a thing above the noise of the wind apart from the loud beeping as transmissions were made and recieved on Ch14. Why does it beep so much and what does it mean? Do all VHF radios do this? Is there a way to switch off the beeping apart from increasing squelch?

 

I couldn't fit a fixed set on my boat as the steering position is at least 5ft away from the stern doors, but my handheld does take an external antenna.

 

The beeping as far as I recall could be atributed to the 'Auto roger' function many sets now seem to come with.....for Auto Roger read auto 'Over'.

 

On the salty stuff I just squelched out the noise and concentrated on the strongest signals that would affect me

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The beeping as far as I recall could be atributed to the 'Auto roger' function many sets now seem to come with.....for Auto Roger read auto 'Over'.

 

On the salty stuff I just squelched out the noise and concentrated on the strongest signals that would affect me

 

 

This called the "Roger beep" on some / most sets I think, and should be able to be disabled...

 

Nick

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The 5/8's aerial is a 1/2 wave, with the first 1/8th of a wavelength wound into a coil to help match the aerial to 50 ohms. So the length of the whip is going to be approx 2X the length of a 1/4 wave, and the coil is of approx length of 9.5", coiled into 3 or 4 turns at the bottom and pulled out to give wide spacing - this is likely to be in stainless steel anyway and not alterable - that's fine - it will be compensated for adequately by the length trimming.

Thanks for comprehensive reply Nick.

 

I'm working with a 5/8 wave 2-metre antenna - the only one I have from years back, although I also found a 1/4 wave and a 7/8 wave, which are less suitable sizes, but possible reserves.

 

The bit I can't see is what there is by way of coil in the sealed plastic part at the bottom.

 

Here are some pictures.

 

I was never sure about what those short stubs part way up were meant to achieve....

 

Dismantled, with mag mount

 

Antenna1.jpg

 

Base part on mag mount

 

Antenna2.jpg

 

Most of it

 

Antenna32.jpg

 

It measures at it's current attempt at cutting for Marine, about 1006mm for the whip and all the exposed metal that goes into. The plastic base brings it up to 1065 mm.

 

On your basis that the whip ought to be about 1/2 wave, (presumably minus the 95%), with the remaining 1/8 wave loading in the base, then I calculate that it should be

 

Full "in air wavelength" / 2 * 0.95

 

Which is

 

1913 / 2 * 0.95 mm

 

But that's only about 909 mm, nowhere near the current length of 1005 mm, despite me already having cut off far more than Allan suggests.

 

So still something is still wrong somewhere.

 

That seems to give an SWR of under 1.3:1, which if correct, ain't bad, but my arrangements for measuring this are a bit dubious given the mixture of connector type involved!

 

I tend to feel I have little choice to use what seems to be correct by experimentation, as I can't see how I can calculate, when I don't actually know what's in that plastic base. I'm just surprised it seems so far adrift of any theoretical calculation.

 

I guess a very low SWR isn't that vital, as for the uses I'm going to put it to, so little time should be spent on TX, with 99 point something being RX, for which I suspect the antenna is more than adequate, given it's replacing a hand-held's rubber duck.

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Simplest thing top do

Take a piece of coat hanger about 20" long, solder it into the centre of a pl259 cut to required 1/4 wave length put that into the mag mount and Robert is your fathers brother.

Its what I use as a backup antenna if the main one fails.

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The beeping as far as I recall could be atributed to the 'Auto roger' function many sets now seem to come with.....for Auto Roger read auto 'Over'.

 

On the salty stuff I just squelched out the noise and concentrated on the strongest signals that would affect me

 

The beeping on my Silva happens even while I'm just maintaining a listening watch and not making any transmissions. It can't be squelched out either.

 

Anyway, I don't know how I'm supposed to hear the thing over the sound of the engine. Turning up the volune doesn't help much.

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What / where are you placing the aerial for checking the SWR ?

 

If its on top of the table as in the picture, you won't get a good SWR as it needs the ground plane ( note 1) that it gets when it sits on a car roof - it forms part of the aerial system ! You may not get much below 1.5 :1 anyway as a quarter wave is not a 50 ohm aerial anyway, so won't properly match the supplied 50 ohm co-ax and output of the rig. (But it's good enough). You must SWR it on e.g. the car/boat roof and otherwise out in the clear, not in the corner of the dining room !!

 

For marine use, a 1/4 wave mag-mount is fine for short range ( a few miles ) but for better range I would highly recommend making a J-stick, which is a an aerial made from thick ( fencing type) wire or small diameter aluminium or copper tube ( 6mm with a 2mm hole through middle). http://www.hamuniverse.com/jpole.html

 

This is similar in operation to the the Slim Jim, I was on about a few days ago, and both have the following very useful characteristics... the radiator is a half-wave, so the main signal lobe is at right angles to it, and the matching can be made very efficient and independent of what it is sitting on or attached to by use of a matching stub incorporated into the lower part.

 

This can be pole mounted, the higher the better, and it lends itself to being mounted on a pole if made from light materials, but good results will be achieved if mounted with the base not far above the boat roof. This should give double the range and perhaps more compared to a 1/4 wave, and will cost between nothing and a few quid and an hour or two. It can be mounted in suitable plumbing tubing ( best white as grey / black may have an effect on the aerial - if in doubt, out a small piece of the tube in the microwave for a few seconds next to a glass of water, and see if it gets warm... if it does, its no good.

 

On your aerial, I am not sure what the 3 spikes do about 1/2 way up it - some sort of loading perhaps to make the aerial a bit shorter than it would have otherwise been..

 

More food for thought I hope

 

Nick

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What / where are you placing the aerial for checking the SWR ?

Mag mount, originally set up in middle of roof of large estate car, but then moved to boat roof to try and get it more accurate.

 

The pictures were taken at home, just to show what the antenna is like, not where it was actually tried.

 

One thing I have found is that the cable I used between the TX and the SWR/PWR meter, which originally had moulded BNCs at both ends, but I replaced one with a BNC, is actually a RG59 cable, so it's a 75 ohm cable not a 50 ohm.

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Mag mount, originally set up in middle of roof of large estate car, but then moved to boat roof to try and get it more accurate.

 

The pictures were taken at home, just to show what the antenna is like, not where it was actually tried.

 

One thing I have found is that the cable I used between the TX and the SWR/PWR meter, which originally had moulded BNCs at both ends, but I replaced one with a BNC, is actually a RG59 cable, so it's a 75 ohm cable not a 50 ohm.

 

 

That won't help ! Everything MUST be 50 ohms, ( e.g RG58 if this size) assuming the rig is 50 ohms ( never seen one that isn't in common consumer transmitters) and the connectors must also be 50 ohm ones ( they look the same) else a poor SWR and performance is inevitable. I prefer to use the tinned copper braid and cable with a tinned and stranded centre core ( solid core is available (UR43) but I fear it wont last in a flexing situation - probably not meant for anything other than a fixed instal - it is fractionally better / lower loss....

 

I struggle to believe you did that Alan, as a fellow Ham ( I think I remember ?). If in doubt, get a couple of new connectors and make up a new lead with new cable ( 50 ohm !!!) It may be the aerial is actually OK...

 

 

Also, fro an earlier post, you can't measure up aerials and expect them to measure up as the theory says it should - they do all sorts of "things" in the mechanical design for a particular purpose, at generally the cost of perfomance, but it does something else "better", allegedly ! :lol:

 

Nick

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If in doubt, get a couple of new connectors and make up a new lead with new cable ( 50 ohm !!!) It may be the aerial is actually OK...

OK,

 

Replacement lead made to go between TX and SWR meter, with correct 50 ohm RG58U cable rather than the 75 ohm RG59U I had inadvertently used.

 

It has made no discernible difference to the antenna tuning, and the antenna still appears correctly tuned at a lot less length than I would have expected, (and a lot more cut off than Allan said!...)

 

Anyway the meter says its OK, so I guess I'll just have to assume it's OK.

 

Just need a chance to try it in anger now!

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  • 2 months later...
Could you recommend a good but cheap marine vhf aerial for nbs.

 

Snip

 

I've just bought one of these. I haven't tried it in anger yet. It does have a (bit too) long length of co-ax but it comes with a uhf (BNC) to vhf (PL259) adaptor. Even has a chart to tell you how long to trim the whip to.

Edited by Ray
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