( PAGE IS UNDER CONSTRUCTION)
LET US INTRODUCE THE MAST AND COAX.
The influence of a mast and coax is something which is underestimated for the vertical antenna.
On this page we will try to visualize the mast and coax is actually part of the antenna and provide data on the behaviour of the mast and coax on our antenna system with various antennas like a 1/4 wave groundplane a halve wave end-fed and a 5/8 wave vertical the collinear and Imax.
Hoping at the end you are able to understand the mast (and coax) can be part of the antenna system and can be one of the reasons why people experience different results with various vertical antennas. Perhaps it will also contribute to your situation which will make you able to “optimise” the entire system.
The visualisation:
Maybe you havnt really thought the mast can become part of your system, so lets try to get a better picture of that.
I would like to do that by asking you to imagine a normale vertical dipole antenna.
The antenna is a halve wave length, has two legs of approximately 2,75 meters and is known to be a good antenna.
If we bring the feedpoint down to the bottom we have an end-fed vertical as often used on 27 MHz. (Antron 99 / GPA etc)
Both will look something like this:
So what will happen if we make that dipole a full-wave ?
something like this:
Well, the antenna will still radiate, we actually might get some gain as the antenna is bigger !
We can expect the antenna pattern may change (if gain changes…the antenna pattern always changes)…and the impedance will have changed.
But …the antenna will radiate (actually with more gain!) and with the right coil..it can provide a good SWR.
Those three will look something like this:
(click to enlarge)
Here comes the clue:
Imagine that full wave vertical dipole antenna isnt completely in the air… but the bottom part is just touching the ground, or the antenna is perhaps just at ground height but isolated from the ground maybe it attached with some wallbrackets to a wall or something.
And lets forget it was meant as dipole…but lets say:
We use that bottom leg as mast and the top section (5,5 meter) is named an (end-fed) vertical….the antron 99 for example.
There is from an electrical point of view no difference… the mechanical and electrical properties of the antenna have not changed, only the names of the structure has.
The result of those antennas are :
(click to enlarge)
That is exactly what happens:In this case your (isolated) mast and/or coax will be part of the entire antenna system and can have a hugh influence on the performance.
In some situations it could provide MORE gain, but it could also SEVERELY DEGRADE the performance.
Your “signal” has no idea what an antenna is, it just wants to radiate !
And it will use any metal object that crosses its path.
Our job is to guide that signal to the path we want (the antenna alone) …so we have optimal radiation.
We can reduce the influence the coax by using a RF choke to illuminate your coax of becoming part of the radiating system.
We can reduce the influence of the mast by changing its electrical lenght, in that way we “decouple” the mast to the antenna.
We change the length of the mast so our “signal” doesnt “fit” on it.
We can do so by perhaps not grounding it or to do the opposite (depending on length) and ground the mast to earth.
Another way to decouple the mast from the antenna is by using a vertical antenna with radials. (and lots of them).
There are other ways to “detune” the mast but that is perhaps a bit to far now.
I am hoping with the above we reached our goal and the reader is able to agree the mast and coax length are part of the antenna system.
HOW MUCH EFFECT DO WE HAVE ?
We will start by look at a traditional groundplane antenna with 4 radials.
I will post the antenna with a mast grounded to earth, secondly with a mast which is isolated from earth and finally with a non conductive mast.
The groundplane at 2,75 meters high, above average ground conditions with the various mast types:
(click to enlarge)
The groundplane antenna at 5,50 meters height above average ground condtions with the various mast types
(click to enlarge)
The groundplane at 8,25 meters height above average ground with the various mast types:
(click to enlarge)
The groundplane antenna at 11 meters height above average ground condtions with the various mast types:
(click to enlarge)
CONCLUSION FOR A 1/4 WAVE VERTICAL
I think we get the idea, we can see that for each time the antenna is posted at a halve wave length or any multiply of that the “best” performance can be expected if the mast is isolated from ground.
And each time the mast heigth is a 1/4 wave or uneven multiply of that (3/4 wave etc) “best” performance occures when the mast is grounded.
The difference between those those ideal situations can vary with several dB’s !
The halve wave vertical at 2,75 meters height above average ground with the various mast types:
(click to enlarge)
The halve wave vertical at 5,50 meters height above average ground with the various mast types:
(click to enlarge)
The halve wave vertical at 8,25 meters height above average ground with the various mast types:
(click to enlarge)
The halve wave vertical at 11 meters height above average ground with the various mast types:
(click to enlarge)
THE 5/8 WAVE VERTICAL:
(click to enlarge)
(click to enlarge)
(click to enlarge)
(click to enlarge)
PAGE IS UNDERCONSTRUCTION 7-4-2025