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Old 07-10-2011, 15:36   #1
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Virgin at Tuning Masthead VHF Antenna . . .

Howdy,

As some of you may know, I have my mast back in. Yay.

This means that I can finally wire up my VHF.

I have a small masthead antenna and low-loss RG213-U to the mast step (keel stepped mast), and then another length of RG213-U snaking around the boat to the radio itself.

If it matters, I have a Simrad RS87.

  • I have a short length of cable at the radio to play with - maybe only 30cm (1ft)
  • I have around 2m of additional cable at the mast step.
  • Total cable run is almost 30m (90ft)

Regarding tuning, I have ordered an SWR meter, which should arrive in the next week or so.

As far as reading various guides on antenna tuning (almost all of which are for CB radios.. and they all suggest trimming the antenna... ouch!)... it appears that the steps are:

1. Connect SWR meter inline.
2. Choose a channel in the middle of the VHF range and transmit.
3. Look at SWR reading and if over 1.5, remove connectors, cut some cable off, reattach the connectors.
4. Keep trying until you get below a reading of 1.5 or until you run out of cable.

OK, so my questions are:

  1. Does is matter where the SWR meter is connected? All guides assume connection inline AT THE RADIO with a patch cable, but can I connect it AT THE MAST STEP (mid way along the cable run)? This is much easier access and would require no patch cable.
  2. If the meter must be at the radio with a - say 1ft - patch cable, do you keep trimming until reading is OK, then remove the meter and patch cable and then cut off an additional 1ft to compensate for the patch cable?
  3. How much do you cut off each time while tuning? Are we talking 1cm, 2cm, 1", 2",6",1ft?
  4. When putting connectors back on each time after cutting, do you have to solder the lot back together each time or just "handle gently and make do"? (I'm guessing this is a no.. but you never know)
  5. Assuming that I still have a pile of cable left at the mast step when I get a good reading, can I coil up this remainder, cable tie it and hide it or is coiling a bad thing? If coiling is bad, should I cut down dramatically leaving only 1ft or so prior to starting the tuning/cutting process?

Thank you copiously in advance!
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Old 07-10-2011, 17:20   #2
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Re: Tuning masthead VHF antenna virgin...

G'Day AK,

I think that you have some misinformation here. One does not "tune" the co-ax transmission line in such an application. Changing the length of the co-ax will not change the SWR. Just hook it up and go. Checking the SWR is ok... might show up a bad bit of co-ax or a shorted connector. Nor is trimming the whip required, for the manufacturer has done all of that for you.
However, you might cut off any real excess length of co-ax since there are line losses even in good cable.

This is a job that you can cross off the list without breaking into a sweat!

Cheers,

Jim

PS: Will be back on the lake on 9 Nov, hope to be in Pittwater by early Dec.
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Old 07-10-2011, 18:19   #3
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Re: Tuning masthead VHF antenna virgin...

Jim is correct, one doesn't "tune" the coax length for a VHF installation, one just tries to keep it as short as practicable to reduce losses; the antenna itself should be manufactured to be the correct size.

But having a suitable SWR meter on board is a good thing!

OK because you seem new to RF, antennas etc, I have to ask, is the your antenna designed as a marine VHF mast top mounted antenna and do you have the installation instructions or some evidence that it is suitable for your application.

You don't want to go to all the work of sticking it on top of the mast just to find out later it was (say) a 27 MHz marine CB antenna or ground dependent VHF or some other unsuitable antenna. FWIW, if one does have to trim an antenna, it is the actual element of the antenna that is cut and yes, it is cut in small steps say 1/4 inch but this is not the place to go into unnecessary detail about stuff that a RF virgin doesn't need to know (yet).

Feel free to ask more questions, we like to help virgins gain experience
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Old 07-10-2011, 18:36   #4
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Re: Tuning masthead VHF antenna virgin...

The less cable the better. Make the SWR test so that you know the installation is fine. Then make a real life test with a boat at a distance.

b.
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Old 08-10-2011, 00:23   #5
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Re: Tuning masthead VHF antenna virgin...

Quote:
Originally Posted by Jim Cate View Post
G'Day AK,

I think that you have some misinformation here. One does not "tune" the co-ax transmission line in such an application. Changing the length of the co-ax will not change the SWR. Just hook it up and go. Checking the SWR is ok... might show up a bad bit of co-ax or a shorted connector. Nor is trimming the whip required, for the manufacturer has done all of that for you.
However, you might cut off any real excess length of co-ax since there are line losses even in good cable.

This is a job that you can cross off the list without breaking into a sweat!

Cheers,

Jim

PS: Will be back on the lake on 9 Nov, hope to be in Pittwater by early Dec.

Hi Jim,

Are you serious?!?!? You mean this is a job on the boat which isn't going to take at least a week?!? No way!

So I just trim to the shortest/nicest length and put the connectors on... and that's it?

I've been avoiding this since there is no way I can replace the cable without unstepping the mast again. The exit out the side of the mast step is barely large enough to pass through all the new goodies I installed on the masthead, so hauling a replacement if I cut too short is not an option.

Great news on your return - will send you an email shortly.


AK
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Old 08-10-2011, 00:31   #6
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Re: Tuning masthead VHF antenna virgin...

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Originally Posted by Wotname View Post
Jim is correct, one doesn't "tune" the coax length for a VHF installation, one just tries to keep it as short as practicable to reduce losses; the antenna itself should be manufactured to be the correct size.

But having a suitable SWR meter on board is a good thing!

OK because you seem new to RF, antennas etc, I have to ask, is the your antenna designed as a marine VHF mast top mounted antenna and do you have the installation instructions or some evidence that it is suitable for your application.

You don't want to go to all the work of sticking it on top of the mast just to find out later it was (say) a 27 MHz marine CB antenna or ground dependent VHF or some other unsuitable antenna. FWIW, if one does have to trim an antenna, it is the actual element of the antenna that is cut and yes, it is cut in small steps say 1/4 inch but this is not the place to go into unnecessary detail about stuff that a RF virgin doesn't need to know (yet).

Feel free to ask more questions, we like to help virgins gain experience
Thanks for the info!

All very interesting! I had thought that the standing wave set up along the length of the entire cable run as the signal "bounced" off the end of the antenna and hence why one could cut the cable to optimize the standing wave etc...

So just to confirm, I can also cut the - too long - lead on my AIS antenna and live happily ever after?

What about my external cellphone/3G antenna cable?

(as you can see, I bought all of this, installed the antennas themselves, but have been too scared to do anything with the cables!

(As for the masthead antenna, I'm pretty sure it's for masthead VHF use, the rigger supplied and installed it for me and he's done a tonne of these.)
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Old 08-10-2011, 00:35   #7
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Re: Tuning masthead VHF antenna virgin...

Quote:
Originally Posted by barnakiel View Post
The less cable the better. Make the SWR test so that you know the installation is fine. Then make a real life test with a boat at a distance.

b.
Well, I've ordered and paid for the meter already, so I guess I should at least give it a go!

Will report back on the reading in a couple of weeks when the meter arrives (coming from the US).
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Old 08-10-2011, 07:09   #8
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Re: Virgin at Tuning Masthead VHF Antenna . . .

Leave a little bit of extra length in the cable in case you need it later. The cable could get pinched in some way, or worn through at some point and you may need to cut out a piece. It is nice if you have a bit of extra length.
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Old 08-10-2011, 08:06   #9
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Re: Virgin at Tuning Masthead VHF Antenna . . .

G'Day AK,

For the other co-ax fed antennas the same rules apply.

On length, if your VHF is flush mounted it is a good idea to leave enough cable so that you can pull the box out of the panel without disconnecting it. Really helps if you ever need to work on something in it.

Cheers,

Jim
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Old 08-10-2011, 08:45   #10
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Re: Virgin at Tuning Masthead VHF Antenna . . .

Just remember to put in a drip loop at the masthead, and leave a bit extra at the base so you can cut the cable back and put in a new connector if yours gets wet and corrodes.

Yes, you can put the SWR meter at the base of the mast to check things out, but the antenna should come precut to give you good performance at the marine VHF frequencies. The best and simplest check is to talk to another boat which is 20 miles away (with no hills between you).
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Old 08-10-2011, 08:53   #11
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Re: Virgin at Tuning Masthead VHF Antenna . . .

After a little on line research it seems that the most important part of an VHF radio and antenna system is the antenna. See: http://www.saltyjohn.co.uk/resources...stallation.pdf

As to the type and length of the antenna cable between the radio and the antenna, indeed it seems the length has very little, if any effect except that the total length is a factor due to line loss - loss of the strength of the signal leaving the radio as other posters have said.
- - However, choice of cable and its installation is very important. Water from rain or salt spray can work its way into the masthead end of the cable and "short" the signal to ground. So waterproof cable or careful installation including a "drip-loop" in the cable at the masthead seems to be very important.
See: #6
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Old 08-10-2011, 09:01   #12
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Re: Virgin at Tuning Masthead VHF Antenna . . .

Dont cut any cable if it already has connectors.Just plug it to the VHF and try to determine the performance.If it has poor performance it is likely to be a connection rather than the cable.Even if there are several feet of extra cable you will not be able to hear any improvement if you cut them out.A bit of extra cable in any installation is a good thing.
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Old 08-10-2011, 09:21   #13
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Re: Tuning masthead VHF antenna virgin...

Quote:
Originally Posted by akio.kanemoto View Post
.....All very interesting! I had thought that the standing wave set up along the length of the entire cable run as the signal "bounced" off the end of the antenna and hence why one could cut the cable to optimize the standing wave etc...
Just the opposite.
Here is Standing Wave Ratio (SWR) 101.
In the perfect setup, you have no standing wave, zip, zero, none. In a less than perfect setup, you will have some SWR. On your SWR meter, perfect is a SWR of 1 (actually 1:1) and anything larger than 1 is less than perfect; however any number under 2 (actually 2:1) is considered OK; A SWR below 1.5 pretty dam good. A SWR between 2 & 3 is very marginal and anything over 3 is very bad. Figures of 5 through to infinity tells you that you have a major fault in the antenna system.

How it works is like this - your transmitter puts out (transmits) RF power, this is carried to the antenna via a transmission line (coax cable) where it is radiated into the universe as Electro-Magnetic Radiation (radio waves).

In the perfect setup, all the RF power produced in the transmitter is converted into radio waves by the antenna. However if the transmission line (coax cable / connectors etc) or the antenna is less than perfect, then some of the energy produced by the transmitter is reflected back towards the transmitter at the point where the system was less than perfect (crook connector / untuned antenna etc). This reflected energy sets up a standing wave in the system. The bigger the mismatch, the bigger the standing wave. This reflected energy is wasted (turned into heat), i.e. it is not being transmitted into the universe by the antenna. The SWR meter measures this standing wave.

So to recap, no standing wave (SWR of 1:1) means all the transmitter power is radiated by the antenna. A small standing wave (SWR of less than 2:1) means that most of the transmitter power is radiated by the antenna but some of it has been reflected back towards the transmitter. A medium standing wave (SWR between 2 & 3 :1) means a fair bit of the power is reflected back and so wasted while a high SWR (>3 : 1)means that bugger all power is radiated by the antenna.

Things that induce this unwanted reflection include crook connectors, sharp bends or kinks in the coax, shorted or open circuit coax conductors, broken or badly bent antenna, incorrect length of the antenna element, corrosion in a connector - the list is endless.

A companion tool to the SWR meter is a 50 ohm dummy load. This item replicates a perfect antenna (without radiating any RF) and is very handy when used in conjunction withe SWR meter. It gives you a known good termination to use with the meter. The use of it is described in SWR measuring techniques 102

If you have digested all the above then you are no longer an antenna virgin...
but you will have a way to go before claiming to be experienced in the black art of RF.
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Old 08-10-2011, 09:26   #14
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Re: Virgin at Tuning Masthead VHF Antenna . . .

Quote:
Originally Posted by osirissail View Post
After a little on line research it seems that the most important part of an VHF radio and antenna system is the antenna. See: http://www.saltyjohn.co.uk/resources...stallation.pdf

As to the type and length of the antenna cable between the radio and the antenna, indeed it seems the length has very little, if any effect except that the total length is a factor due to line loss - loss of the strength of the signal leaving the radio as other posters have said.
- - However, choice of cable and its installation is very important. Water from rain or salt spray can work its way into the masthead end of the cable and "short" the signal to ground. So waterproof cable or careful installation including a "drip-loop" in the cable at the masthead seems to be very important.
See: #6
Indeed you are correct sir.
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Old 08-10-2011, 09:37   #15
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Re: Virgin at Tuning Masthead VHF Antenna . . .

SWR generally should be measured at the radio.

The coax length does indeed have an impact on SWR, though at VHF frequencies with good coax the impact is usually slight.

That said, the overall length of coax for VHF installations generally matters little, except for attenuation at VHF frequencies. That's why it's a good idea to keep the length as short as possible -- with a service loop for later replacement of UHF connectors or for servicing the radio -- and to use good quality coax which has relatively low loss at VHF frequencies -- like RG-213 or, better, LMR-400. If you MUST use smaller diameter coax, RG-8X is the way to go.

Bill
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