Here is my take on GP for 30m as a simple to make and effective ANT.
Advantages:
- low angle take-off good for DX
- easy to make, low cost
- resonant (single band)
Simply this antenna requires a vertical element one quarter wavelength long and if ground mounted, a good network of buried ground radials, or if elevated, 2 or more radials, 3 or 4 is ideal, each about a quarter wavelength long.
I do not recommend ground mounted GP unless you are out in the open in a nice field.
For most of us in suburban areas the ground mounted GP is not going to perform well due to absorption and blocking within several wavelengths of the antenna due to surrounding shrubbery and buildings. Hence, the elevated GP would be what suits most of us.
Elevated GP does not need to be high off the ground, I suggest going with whatever is practical, for example, roof mounted is a good option for many.
In my opinion 3 or 4 elevated radials, i.e. on an elevated GP is as good as dozens of buried radials, and even better in most situations where you don't have a clear view to the horizon at ground level.
Although 1 radial would suffice to make it resonant and work, 3 or 4 radials will produce much better results, and if roughly spaced in all directions, an omnidirectional coverage. If you use only one radial it will favour the direction of that radial.
If the radials slope downward say at an angle of 45ยบ the impedance will be around 50 Ohm as opposed to 70 Ohm or so if they are horizontal at right angles to the vertical radiating element.
The vertical element is connected to the end of the coax inner while the radials are all connected to the coax outer.
This antenna does not require any additional grounding or earth at all.
On 30m this means each element of the antenna is going to be roughly 7 or so metres long.
If you don't have the vertical space, or, for example, a pole only 5m long, you could try a couple of things: one would be to loosely wind in helical fashion the wire so it fits on the pole, or you could make a coil at the base and wrap several windings of the wire around it: note that on these suggestions I do not have much experience or knowledge, so you should research and experiment.
You can also make the vertical section longer and the radials shorter, adjusting for SWR, or conversely you can make the vertical section shorter and the radials longer, adjusting for SWR.
Such modifications will reduce the efficiency or effectiveness of the antenna but can be a better option than not getting on the air at all, depending on your circumstances. See other antenna options in this forum. Feedback and experiences from 30CW members is most welcome.
It is often said that verticals pick up more noise in suburban areas, compared to horizontals. This is by and large true though it can depend on where the noise is coming from. It might be worth adding a line isolator near the TX in case any noise is being picked up on the coax.
Generally a GP is a mono-band antenna. However it may work well on adjacent bands with or without a tuner, and is worth experimenting. For example, a 17m GP can work well on 20m is my experience, conversely a 20m GP may work well on 17m.
The (elevated) GP is a simple antenna to make, and is a good performer given its low mounting height, though in my opinion it helps to raise it above surrounding objects.
Most of the radiation is coming from the LOWER to MID section of the vertical element, the top is not doing anything much, just as with the ends of dipoles.
To my mind this means that you could also, instead of the earlier suggested options of helical winding or base loading coil (both of which would reduce efficiency) perhaps do something else if you won't have enough vertical space, and that is to pull the top of the wire away from the pole.
For example, say the vertical wire length is 7.5m and you only have 5 m space. Run the wire up the bamboo, or if a metal pole, add an extension at the top, and the run the wire out in a convenient direction for tie off from the top, so that in effect is looks like an inverted L.
In my opinion this may actually help raise the radiating portion higher and act as a capacitance hat. I'm using this I believe to quite good effect for the same reasons in a vertical inverted L end fed half wave via 49:1 balun, which will be the subject of another post.
I hope this was useful or helpful to you and you have any comments or corrections to make.
77!