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Tech808
Moderator
Username: Tech808
Post Number: 10898
Registered: 8-2002
| | Posted on Tuesday, August 22, 2006 - 2:03 am: | 
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Moderator Note!
I am posting this Article for Simon ~ Tech237
Coax Cable's
Facts and Fantasy
An Article Written
By
Tech237 ~ Simon
Over the year's a large number of myth's and fantasies have appeared concerning coax cable's, their use and how and why they work. It is my hope to dispel some, if not all of these, and to give you a better understanding of this style of feed line.
What is Coax?
Now despite how it look's coax is more than simply two wire's running one inside the other, it is in fact a complex series of inductor's in series, paralled with capacitor's.
Any single piece of wire has an amount of inductance but at low frequencies (DC up thru audio frequencies) this is usually so small that it is ignored.
Coax is normally used to carry RF, and here, this inductance and capacitance actually become important.
The capacitance of coax comes about due to one conductor being run inside the other and is generally expressed in terms of pF per foot.
It is this combination of inductance and capacitance that give's coax its characteristic impedance (Z). As an aside, due to this inductance and capacitance coax has even been used to make loading coils and traps for antenna's.
How does this inductance and capacitance effect using coax as a feed line?
This is not a simple question to answer as several other factor's are become involved.
If the coax was infinitely long, both the inductance and capacitance could be safely ignored. Unfortunately coax feed line's are of a finite length and are terminated with a load – usually an antenna of some form.
If this load is purely resistive, and matches the characteristic impedance of the coax, the current “see's” the load as being a continuation of the coax. When this condition occur's the coax is said to be a matched line and the source of current is presented with impedance that remain's the same regardless of line length. This impendence is of course the characteristic impedance of the coax.
If the terminating resistance is not equal to the impedance of the coax, the load no longer look's like a continuation of the coax and we are now presented with a mismatched line.
Some of the power reaching the load is no longer absorbed by the load and is reflected back toward's the source. The amount reflected depend's on how much the load and coax are mismatched. The greater the mismatch the greater the amount of power is reflected.
Very few antenna's are purely resistive as load's and this non-resistive value change's as the antenna get's further away from it's frequency of resonance.
This reactive component affect's how the source and coax see the load. Like a resistive mismatch the reactive one will cause some of the signal to be reflected back toward's the source and again how much is reflected depend's on how bad the mismatch is.
Lets look at the two worst cases of mismatch – a totally open circuit load and a shorted load- neither of which will deliver any power to the load. In both case's what the load see's now depend's on the length of the coax cable.
All feed line length's that will be used are the electrical length of the line and not the actual physical length of the wave. The difference in these length's depend's on the difference in time that it take's a signal to travel one wavelength in the feed line when compared to the same signal traveling in free space.
This is called the Velocity Factor and in most 50 ohm coax is around 0.66. From this you can see that an electrical length of the coax will be shorter than the same length in free space.
If the coax is shorted at the far end and the length is equal to ¼ wavelength it will have the properties of being a resonant circuit with a high impedance (parallel tuned circuit) if short-circuited and a very low impedance (series tuned circuit) if open circuited at the resonant frequency.
If the line length is different from a ¼ wave the line will have either inductive or capacitive reactance depending on the length and how it is terminated. If short-circuited it will be inductive and an open circuit coax will show as capacitive. There is a formula to work out what the exact value is but I’ll leave that for another day.
Bear in mind that although neither of these extreme's make's a successful feed line they both can be put to use in antenna's as circuit element's.
Can Coax Be Used to Tune Antenna's?
Now our next fact or fantasy concern's the often stated ability to use coax to tune an antenna.
Let me state here and now that nothing you do to coax used as a “simple feed line” will actually effect the tuning of any antenna, the only place to tune and antenna is right at the antenna.
Where the confusion come's in, is some commercial and homebrew antennas which are designed to use a certain length of x impedance coax as part of the feed line. In these case's the coax is acting, not only as a feed line but also as a circuit element that is being used to counter-act (or tune out) the reactive impedance that is inherent to that antenna.
This is why some antenna's have a length of 75 ohm coax as part of the feed line. Although in this case it act's more like a transformer. You are still not using the coax to tune the antenna just removing the reactance and transforming the impedance so that proper adjustment's at the antenna, now can tune the antenna to the frequency(s) in use. If that reactance was allowed to remain the antenna may be difficult or even impossible to tune on some frequencies.
Like tuning the antenna itself, the best place to check to see if an antenna is tuned correctly is right at the antenna. It is understood that this may not always be feasible. In the past I have spent many an hour hanging off 100ft plus towers tuning antennas and it not always fun. In most case's we check our antenna tuning in the shack – but is this an accurate check?? Unfortunately the answer is not a simple yes or no but an unqualified Maybe.
Remember that earlier, we mentioned the reactive impedance that exist's in length's of coax (by the way other type's of feed lines also suffer from this) that are less than a multiple of a ¼ wavelength? Well this reactance can affect the reading you will see in your shack. That 1:1 or 3:1 SWR you see on your meter next to the radio may not be the same reading you’d get if the meter was connected directly to the antenna – it depend's on the length of coax and how much reactive impedance your feed line/antenna has.
To try to put it simply – if you feed line is an odd multiple of a quarter wave your radio sees the antenna as resistive value higher than the true value of the load resistance and if your feed line is an even multiple of a quarter wave your radio sees the actual resistive value of your antenna. Any feed line that is not either of these cases will still show the load resistance to be higher than it really is, but now the reactive component is also involved. Now any load that is purely resistive will always have a lower SWR than the same load would have if a reactive component were added. Trying to work out the actual effect fro a random length of feed line would be extremely difficult.
From this we can see that using an electrical half wavelength of coax does not actually tune you antenna in anyway but does allow you to see the SWR more accurately in your shack.
As an side, not only are calculated length's of feed line used to help match antenna's by tuning out the reactance, they can also be used to join multiple antenna's to a single feed line and this can be observed in phased array of antenna's.
Respectfully Submitted,
Simon
Tech237 |
Moderator476
Moderator
Username: Moderator476
Post Number: 52
Registered: 7-2006
| | Posted on Tuesday, August 22, 2006 - 2:20 am: |
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Thanks Simon I now understand Coax a lot better
Moderator476 |
Bruce
Senior Member
Username: Bruce
Post Number: 4001
Registered: 9-2003
| | Posted on Tuesday, August 22, 2006 - 6:14 am: |
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How many times have I posted on cutting coax to tune things ........
Good Article ...... |
Unit199
Intermediate Member
Username: Unit199
Post Number: 306
Registered: 8-2003
| | Posted on Tuesday, August 22, 2006 - 12:09 pm: |
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AMEN BRUCE......GOOD ARTICLE THOUGH.
HARVE
UNIT199 |
Road_warrior
Senior Member
Username: Road_warrior
Post Number: 1858
Registered: 7-2004
| | Posted on Tuesday, August 22, 2006 - 5:53 pm: |
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Hi Simon,  |
Chad
Senior Member
Username: Chad
Post Number: 1045
Registered: 11-2004
| | Posted on Wednesday, August 23, 2006 - 1:58 pm: |
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I tune by cutting coax!!!!! I tune the length of the coax so as to not have too much coiled up anywhere, makes for a sloppy tower 
Chad |
Dale
Advanced Member
Username: Dale
Post Number: 886
Registered: 12-2002
| | Posted on Wednesday, May 23, 2007 - 5:24 pm: |
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i use whatever lenth it takes from point a to point b
dale/a.k.a.hotrod
cef426
cvc#64
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