Testing a VHF antennae

[MainlySteam]

is there a relationship between the cable length , the power output and the reading on a SWR meter?

Cable length and SWR

If the cable is lossy then the length of the cable will affect the SWR, however, that does not mean that the SWR changes along the cable. If Glomax are saying that by changing the length of the cable you will improve the SWR then they are providing bad advice because if that does happen then you have an installation fault resulting in the cable radiating (as well as the antenna) . If Glomax are saying that by shortening the cable length you will get more power to the antenna and therefore better performance then they are correct.

The complex impedance (bear with me) changes along the cable in terms of its resistive and complex (inductive/capacitive) components but if you get a better match by changing the length of the coax to get a better impedance match with the radio (again in complex resistance/capacitance/inductance terms) and therefore get its maximum power output then there is really some sort of problem that should be attended to if it is serious enough to greatly affect the output of the radio.

Power Output and the Reading on an SWR Meter

Two things happen when SWR increases from 1:1. Firstly, the output power from the VHF radio into the cable will be decreased by the sets own protective circuitry (although I think pleasure marine ones may not have much sophistication there) and from the impedance mismatch between the radio and cable. Secondly, the losses in the cable increase as SWR increases so less power gets to the antenna.

Checking VHF Radio

I managed around 35 surveyors of ships while recovering a company for a client a few years back. For radio surveys we decided against them making any measurments at all, but if a problem was identified or any doubts then a radio technician should be called in. We did not include SWR measurements because, as has already been seen in this thread, many are confused by what the reading actually means. I do not think my client will mind if I try to post separately after this, because it is long, a copy of the VHF checks we required to be done. The references are in our local terms, but are readily translated into others situation - I suggest that the radio check part should be used sparingly (maybe once every 1 - 2 years unless a problem is suspected). The surveyers were not required to test the DSC functionality of radios so that is not mentioned.

Trust that is of some help

John

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[MainlySteam]

Here it is; channels, documents, etc mentioned should be read as being what ones own local situation is. In the original there was also a warning about exposure to radiation from high powered radios - have deleted that as unlikely to be found on pleasure craft.

VHF Radio

REMEMBER: When transmitting, VHF radios present a burn hazard from the high radio frequency voltages which may exist when transmitting, even at low power outputs. While radio frequency voltages are not normally life threatening they do cause deeply penetrating burns. Care should be taken not to touch any part of the antenna when the radio is operating, even if it is insulated.

* Where accessible, check antenna and antenna feed-line connections for security, sealing against water ingress (eg self amalgamating tape) and condition—it is unlikely that any connectors that are used are inherently weatherproof, special steps have to be taken to seal them. Check that the antenna is mounted as close to vertically as is possible and, if its structure appears to have its watertight integrity broken, require a radio surveyor’s or inspector’s, or radio technician’s inspection.
* Check condition of coaxial cable from antenna to radio (to the extent that it is accessible) and check for damage, including damage to the jacket which could allow water ingress. Check for any evidence of water leakage down the cable shield and out the connector at the radio end.
* Check condition and security of power supply cabling.
* Check that normal functions work (volume, squelch, channel switching), that the push to talk works reliably, that the display is operable, and that the radio goes to Channel 16 when operated in accordance with manufacturers instructions.
* Check radio switches to channel 16 when the channel 16 priority select key is operated.
* Check radio switches to all ITU channels. If it is not designed to cover all channels, that it switches to at least channels 6, 16, 67, 68 and 71 (or other mandatory channels as may become required by the Maritime Rules).
* Check that the radio operates.
* Have the ship’s operator obtain a signal report from a distant shore station, calling first on Channel 16. A 25 watt radio should be able to easily work stations 20 miles away over water, and significantly further with suitable ship’s antenna height and shore station elevation (around 50 miles or more if working the MSA coast stations). A 5 watt hand-held radio should easily work over line of sight paths, and at 1 watt output radios should easily work bridge to bridge. Coverage diagrams for Maritime Safety Authority coastal stations are in Part 43 of the Maritime Rules, in the New Zealand Nautical Almanac, and in the MSA booklet Radio Handbook for Coastal Vessels (previously titled Small Ships Radiotelephone Service - General Instructions). In New Zealand all Maritime Safety Authority and most other coast stations will give a signal report if requested.
* Check condition and security of reserve electrical power supply cabling and batteries. For hand-helds, if there are no charging facilities on board, ensure that spare batteries are carried.
* Check that the station clock is operable.

John

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[ean_p]

Many thanks for your reply John. Clearly you are on your subject and that been so let me pick your brains a little so as to clarify and or dispel my views which I'm beginning to believe are incorrect.
When you say that by shortening the cable it would increase the power output at the ariel, is this because you have shortened the cable per se, the amount being irrelevent , indeed the more the better or have you in some way 'tuned' the assembly. My impression was that the energy moved in a wave form and that the peak of that wave needed in some way to fall at the mid point of the ariel so therefore the cable and ariel length in total needed to be some 'multiple' of that wave length . And should it be that the length was incorrect then the energy sent would fail to be transmitted and in effect be 'bounced' back down the assembly to the radio unit therein possibly damaging the output stage etc and showing up as a high ratio on a SWR meter! Clearly me thinks this might well be wrong now! If you patched a SWR meter in line and it gave a reading of say 2;1, what would you generaly deduce from that and how would you correct it. And finally what does a SWR meter really tell me, what is a 'standing wave'??

hope you don't find answering this to laborious, many thaks in anticipation

Ian P.

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[MainlySteam]

Joe

It may be that you can buy a combiner sold for the exact job. But if not I would try an ordinary TV combiner (sometimes called diplexors or duplexors) used for combining the signal from two TV antennas, VCR, etc into a TV set . These are generally sold without any particular frequency/band type but if asked try for a Hi VHF/Low VHF one but it should not really matter - I see them listed in UK claiming to handle satellite and off air, so not frequency specific.

Particularly ask/look for a "hybrid" combiner and if the salesperson looks blank, they are the ones that have a little ferrite toroidal transformer inside (rather like a TV balun but may be a round ring rather than the flat ferrite with two holes through it) and only cost a few dollars here. Some are made up with resistors instead of using a toroid and are a poor solution.

In the end, one can just switch the antennas close to the receiver - you can buy switches for switching antennas to TV's which should do the job, but making one up with a rotary switch (taking the coax centre conductor and the braid to the relevant contacts) will work but is hard to house in a tidy box.

John

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[MainlySteam]

By Shortening the Cable Increases Power at the Antenna

The only reason is that all transmission lines (eg coax) are lossy so the shorter the better. Also, some transmission lines are lossier than others - as a generalisation for coaxial cables, the larger the diameter the less lossy it is. Also, the higher the frequency the lossier the cable becomes and is particularly important at VHF and above.

So the amount you shorten the cable is relevant - the shorter the better and the bigger the better too.

I would never use the small diameter coax (usually called RG58) for VHF and certainly not to the masthead (the large is usually called RG8 or RG or URM213). Typically, using small coax to the mast of a 40 foot yacht will reduce the radios range by about 20 miles to hilltop shore stations (assuming a 60 foot run to the masthead, more than half the power will be lost in the cable to the antenna; it is lost as heat).

Wavelength and Coaxial Cable Length

Despite myths to the contrary the length of a matched coaxial cable (eg radio output, coaxial cable and antenna all are 50 ohm) does not have to be any particular length in terms of wavelength at all. That is the situation you have with a marine VHF station (but not necessarily with others).

Standing Waves and "Bouncing"

Again, contrary to much myth the waves do not travel up and down the coaxial cable so cannot "bounce" - they are standing waves. There are reflected and incident components at any one point on the line. A standing wave does not travel - most waves in the sea are not standing waves, but the waves on a guitar string are standing waves ie they do not travel up and down the guitar string, but they do have reflected and incident components - quite hard to get ones head around unless have some background in the physics of waves

All the power put out by the radio (but that power may reduce if there are standing waves on the coaxial cable) will be radiated by the antenna (again myth says the opposite to that claiming along the lines of the reflected power bounces back and forwards until lost) excepting only that the losses in the coaxial cable increase with increasing SWR. But this loss is not the "bouncing" back and forwards myth but just the ohmic losses due to the effective increase in current and because the effective increase in voltage results in greater dielectric losses in the insulation between the centre conductor and the braid. The "effective" increase is due to the existance of the waves.

What does 2:1 SWR mean - it is the ratio between the maximum voltage to the minimum voltage on the line resulting from the incident and reflected voltages (VSWR). The SWR is related to the mismatch between the antenna and the coaxial cable impedances according to a "reflection coeficcient" which is a complex number (ie has real and non real parts if you want to be really confused) so not easily described.

If you measure an SWR of 2:1 it means that between the SWR meter and the antenna there is an impedance mismatch somewhere. That can be because of the wrong impedance coax being used (eg the antenna is 50 ohms, as a marine VHF antenna should be, but the coax is 75 ohm), because something is wrong with the antenna so that its input impedance (if marine VHF) has changed from 50 ohms. But without knowing that one or the other of the coaxial cable or antenna is correct one cannot determine which is the faulty one - which means that by measuring the SWR you cannot determine if it is the cable or the antenna or both which are faulty unless you already know one is correct - the implications of which are obvious if our antenna is at the top of the mast and the coaxial cable is inside the mast. You have to get one or the other down if relying on SWR measurements.

Again, despite urban myth to the contrary, one cannot, except in only an indecisive way, determine from measuring the SWR of a piece of coax by itself whether it is lossy or not - if a piece of coax is connected to an SWR meter with nothing on its end then the lossier it is the LOWER will be the SWR measured. If one puts a matched load known to be correct on the far end of the coax then one can make some assumptions about loss but the correct way is to inject power into the coax and measure the power at each end, the difference being the loss.

It is easy to see that in the overall picture one can have an accumulation losses and mismatches all together giving a false impression of SWR. Taking a simple example - there could be a fault in the coax raising its characteristic impedance (the 50 ohm bit) and a fault in the antenna raising its characteristic impedance (again the 50 ohm bit) and if both are raised to the same amount then the SWR will be close to 1:1 even though everything is faulty. Obviously such a close concurrance of impedance change is unlikely but serves to show the point.

If you have 2:1 SWR you will increase the power loss in the coax (I am not working it out for a typical coaxial line but at VHF it starts becoming important from around 2:1) so the power radiated from your antenna will be less, and your radio may also be putting out less power because it does not like working into a 2:1 SWR. Typically to around 1:1.5 or so is is fine.


It is all quite a difficult concept to get ones head around. I have taken some shortcuts and possibly may be little reasons (but not outcomes) not strictly as I say but generally the theme is correct.

So what does the radio surveyor do normally with VHF - he will likely measure SWR (or return loss) but put that in the context of the physical signs he can see that all is in order and by subjective assessment be satisfied that the radio actually performs as he would expect in so far as voice quality and signal strength is concerned. That is, just because the SWR is ok he will not assume that all else is.

John (now seeing double but hope that answers some of your questions)

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[Joe_Cole]

John,

Many thanks. Easier than I feared. I'll try it!

Joe

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[ean_p]

John
Many many thanks for an excellent answer.

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