VHF Aerial

[Aspasia]

I am just about to change my VHF and Aerial. - I understand that radar leads should not be shortened, but does the same apply to VHF leads? - I don't really want miles of cable if I can avoid it. Any advice would be greatfully received.

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[Oldhand]

Yes you can shorten VHF leads, the shorter the better but make sure you know how re-make the connectors and have appropriate equipment to do so.

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[VicS]

When I fitted VHF both the deck plug and the plug at the radio came with instuctions on fitting them to the cable.

I suggest, therfore, that when you buy these you make sure you get instuctions with them.

Btw, Aspasia, welcome to the forum

<hr width=100% size=1><font color=purple>Ne te confundant illegitimi.</font color=purple>

 

[Birdseye]

You can shorten radar leads according to JRC anyway. I did that on my JRC1000 and joined the leads using a chocolate bar connector, again as advised by JRC. No problems at all

<hr width=100% size=1>this post is a personal opinion, and you should not base your actions on it.

 

[capt_courageous]

You can shorten the power supply leads using choc strip if you must - but NEVER use them on the radar RF lead. It could seriously damage the radar output stage due to mismatch.

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[Talbot]

<blockquote><font size=1>In reply to:</font><hr>

<<You can shorten the power supply leads using choc strip if you must - but NEVER use them on the radar RF lead. It could seriously damage the radar output stage due to mismatch. >>

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The Furuno Agents disagree with you. Their major concern was the ability to terminate properly

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[Benbow]

I am staggered to read that. I would never even use chock-block on power cable, especially critical and high current leads like a VHF radio, but the thought of using it on an RF connection makes me feel quite unwell.

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[Talbot]

I will be reducing the cable to the correct length , but will remake the connector to the scanner rather than chopping at the base of the mast. I will then use a through deck gland, and when I need to remove mast, I will decouple from the display and pull it through the deck gland (relatively easy on my boat)

As an aside, I saw a novel method of dealing with the radar cable the other day. The cable had been installed in an electrical conduit mounted on the outside of the mast (certainly simplifies the installation)

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[Tekkie]

If you are talking about shortning the Ariel cable, remember you should "match it" a high Voltage Standing Wave Ratio VSWR will cook your VHF output stage.

Regards

Brian

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[Jools_of_Top_Cat]

The coax cable on a typical marine radar set is actually video not RF.

I had the same assumption before buying my radar assuming the TX to be in the display unit. I wrote a long passage on here advising someone about using the correct coax or else death to the radar.

In hindsight I was wrong, the worst thing you could do to the radar by joining the coax incorrectly would be to degrade the signal (assuming you do not short the cable!)

As to the VHF it is not simply a case of cutting to length and joining up. It is important to keep down the Standing wave ratio. A high SWR can occur when the cable (or aerial) length is wrong. SWR is power reflected from the aerial rather than transmitted or radiated out. Imagine it as an echo, signal goes up cable and echoes back. The stronger the echo the less power to the aerial, as the echo can stop or degrade the transmitted signal reaching the antenna. For example, a bad mismatch could mean a negative wave cancelling a positive wave and therefore zero signal. The worst case being a ¼ wavelength standing wave at the joint it actually makes an electrical open circuit. (I think that is right, could be short, been a while since I have had to use my theory).

As a rule of thumb you need to work out the wavelength of the signal then cut the cable to be a multiple of half wavelengths. The wavelength of 156.8 equates to a wavelength of 1.91m (speed of light divided by frequency or 299792458 / 156800000). A half wavelength therefore being 95cm, so if you wanted a 15m approx cable you should actually cut to 15.78m for example. A good SWR meter would be useful at this point to fine tune the set at Ch16. You are aiming for 1:1 SWR. You will not get this, 2:1 is acceptable, but careful trimming and slight adjustments of the whip aerial should get you close. (loosen Allan screw end move aerial a millimetre, tighten and retry).

To be fair tuning to this degree is a little far fetched, and for general use you will not have to be millimetre perfect, this is just the theory. There are so many other factors that can affect your VHF transmissions, but getting as close as possible with your antenna matching should help and allow other factors to be taken into consideration later.

Changing channel will change frequency and obviously the SWR will suffer as you scroll through. But as 16 is the most important this is the one usually set up.

Connections are very important, use m-f inline bnc or N type connections.
Male

Use gentle curves when feeding the coax through the boat, a kink in a cable will reflect an RF short circuit and the signal reaching the antenna might be negligible. Do not stretch or strain the cable, this might compress the insulator between the inner and outer and degrade performance.
The correct cable either RG-58 for runs up to 20m or RG8 for runs exceeding this is also important to maintain a good signal.

In actual fact the shorter the RF coax to the antenna the better, but taking into account the wavelength when choosing the length. RF cable will have losses.

This is not to be a lecture, sorry if it is long winded, it is a large subject with lots of variables, I may have explained some things incorrectly or confusingly, I am sure I will be corrected in due course.

I hope it is of some help though.

Jools


<hr width=100% size=1>Julian

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[Nick_Pam]

You can't match the antenna cable (feeder), since the whole purpose of it is to present a constant 50 Ohm match to both the radio and the antenna.
You can, however, match the antenna, if necessary, although if you buy a reasonable marine one, it probably won't need adjusting.

Same goes for radar RF cable, so there is nothing wrong with shortening it, but please don't terminate or joint with a "choc-block" as the point of using coaxial cable is to prevent leakage of signal out and leakage of interference in!!! Either you will cook yourself (unlikely I admit!!), or let in so much interference that the picture on the display will be unusable.

Simple rule of thumb is that if you are working with coaxial cable, then use a coaxial plug on it.

Hopefully that helps??

Nick

<hr width=100% size=1>Byeee...Gone Sailing!!!!

 

[Bergman]

Re: Er

I hesitate t throw a spanner in such an impressive post

But

Feeder length is not critical IF the load (aerial) and the source (wireless) are both the same impedance,

Generally speaking the source can be taken as read as 50 Ohm and it will be the aerial that varies. Any reputable supplier will ensure that the aerial presents an impedance close enough to 50 Ohm so that it makes no difference.

I suspect you are thinking about a tuned feeder arrangement in talking about half waves, different arrangement to using co-ax.

A open circuit quarter wave is effectively the antenna that you will be using, it presents a very low impedance and will have some matching arrangement to give around 50 Ohms. Funny things quarter waves. I once operated with an earth lead that was a quarter wave long on 21 MHz, I got lots of RF burns until I twigged what was going on. The point is that one end of quarter wave is high impedance and voltagage the other is low impedance and voltage, so makes a good transformer. An open circuit quarterwave can be used as a tuned stub to block out a particular frequency that causes interference.

I would try to make as few connections as possible and then only use BNC connectors.

Why do so many manufacturers use PLG 259 connectors they're hopeless for VHF.

Suppose I've now bored everyone who you missed





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