VHF antenna - high SWR

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supermalc

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I've got a vhf antenna, and the cable is very short. It appears to have a high SWR with the longish lenth of coax I'm using. Does anyone know the length of the coax required for a whip antenna.

Cutting down a long lenth, half inch at a time, then reattaching the plug would take years /forums/images/graemlins/laugh.gif And I'm still not sure of success.

Ta
 
Any length you like. There is some loss of signal for every metre used, so don't make it too much longer than you need. But there is no 'special' length. High SWR is likely to be the result of a poor connection somewhere or a break in the coax.
In fact I don't think I really understand the question! - you must of course use 50ohm coax and make the connections very carefully.
 
Erm,is the cable short or long?,and what is a "high " SWR reading.?.Are you using a meter capable of giving correct reading on 160 mhz (ish).27 mhz CB types will not give accurate reading altho will give rough idea if OK.
To be brutally honest if range and quality of Tx is acceptable and meter is not showing a horrid "well into the red" reading,just use the thing.
 
Well the antenna has been sent by a friend, and is of unknown origin. I know some 27m antenna had to have 3 meters plus of coax, otherwise a high SWR could result.

I understood a whip antenna was a tuned wound length of wire, which included the coax. I don't have the base (the clamp) would this act as a ground plane and so do I need to fasten it to one. At the moment it is bolted to a long ally pole.

I've no experience of VHF antenna, but loads of CB and ham radio (I'm not a radio ham) just use radio to communicate to others, as that was what they were made for. Today everyone does it, only they are called mobile phones, with the astronomic costs.

As I can hear faint speach from Boston, and I'm at home in Lincoln, it is probably good enough to use.....especially the limited use it'll have, as I suspect I'll only be using it if, or until I sell my boat. May use it on friends boat though.

In the Wash we used our old CBs, which worked brilliantly. Only problem was my friends switch me off /forums/images/graemlins/mad.gif so I couldn't call them, only the other way round.
 
VHF antennas do not require any specific length of cable. If you use RG8X coax, 50 ohm resistance (not TV coax), on runs up to 15m and RG8U thereafter you should be ok on the cable side of things. If your PL259 connectors are not fitted correctly, or you have water wicking into the ends of the cable this will be a problem. Connections are usually the culprit if a good system goes bad. If the cable is good, the antenna is at fault. An SWR meter cannot detect if the fault is in the cable or the antenna so you need to be confident either one or the other is good. An SWR of lower than 2:1 is barely acceptable, 1.5:1 is better, 1.2:1 is brilliant. If the SWR meter pegs, you have a short in the cable or connections - do not operate the radio. If you have an antenna with factory crimped on cable and it has been in use a couple of years it could be the crimped connection, or if you are trying to extend RG58 coax to use on long runs you will get massive losses. Hope this helps.
 
SWR meter is a 27M one, but........all meters, remote oil pressure gauges, temperature gauges etc. etc. are simply voltmeters calibrated differently. This is showing in the red at 1 watt, but also same reading at high (is that 24watts) It is just a very rough guide of comparing the impedience.

I've now fitted a new plug, and get a full circuit connection. Before I was getting a slight resistence.

Yes, thanks, I do intend to just use it, but if it was a simple answer of just cutting a bit of wire (coax) to the correct size I thought I might as well do that /forums/images/graemlins/smile.gif Pretty sure it's ok.
 
Thanks for all your help.

It is an old antenna I've begged. Came off a wreck on the south coast I believe. Can't check the coax to antenna connection, but suspect it's ok (ish) I'm using 50ohm coax with plugs of proper connections etc. As it is a whip (no loading coil) I can't check if there is a leak across the short length of coax left on the antenna, as it just shows full circuit when connected from inner to braid.

The irony is, I usually keep all 'junk' I find, or am given, and last year was given a good antenna. Used the clamp for something else, tripped over the whip so threw it.......typical.
 
i said i would not post radio stuff again, but using a cb swr meter for vhf, a good reading will be 3:1 for vhf on the cb meter.
 
i said i would not post radio stuff again

Given the absolutely misleading and incorrect advice you have just given in this thread, it would certainly be best if you did refrain from posting on "radio stuff".

John
 
if you take a cb swr meter and hook it into a marine/2m antenna line, and the line is good, the cb meter will read a 3:1 match. this is a fact, am sorry if you are not comfortable with the information, try it
 
Bruce,

With the greatest respect, I wish you would stop saying things that are not always true. The reading on the CB meter at different frequencies will depend on the design of the meter. There are various ways of building a Standing Wave Ratio Bridge (as it might more correctly be termed) and even in the same design, the reading can vary with frequency as well as other factors. (Its one of the downfalls of the simpler designs.)

The reading at 156 Mhz compared to 27 Mhz might be different due to the length of the lines in the bridge as well as the characteristics of the diodes. The loads on the end of the lines inside might also have different characteristics and show different impedences at different frequencies.

Furthermore if the length of the coax is altering the reading, then there is a serious mismatch and the coax is not acting as a tranmission line, but a rf line transformer.

If you use a SWR bridge designed for VHF use, as a general rule it will be accurate for lower frequencies. Howver you may have sensitivity problems.

Finally, at lower reflected values, the meter tends to 'under read' due to the characteristics of the diodes that are used. High quality commercial meters get round this by forward biasing the diodes, and using high impedence amplifiers to ensure that the readings are more correct.

And yes I was an RF engineer once.
 
Sorry, it is not a fact at all and if people are mislead by it their radio installation will suffer in performance (and possibly, but unlikely, be damaged). That could obviously lead to a safety issue if the radio has to be relied upon, meaning that your information could place people in harm.

I agree with everything that John says.

(I have a university degree majoring in electronics and work regularly on telecommunications projects, including specifying radio systems for commercial and other vessels).

John
 
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