Calling all antenna experts

[Dr_DX]

Got a question about whether you should or should not ground elevated omnidirectional antennas (that aren't designed to use the mast as part of the antenna).

Copying Bob85's signature:

W8JI
"An antenna with a poor ground using few radials cannot have a support mast grounded to the radial common point (at least it shouldn't if designed properly) There is no exception to this!

Does 4x 1/4 wave ground radial constitute a "poor" ground? The modeling that I have seen would seem to indicate this - i.e. isolating the antenna from the mast shows it to perform better.

What about DC Grounded antennas? If you use a RF Choke at the feed point to eliminate CMC on the braid, you still give DC a path to ground.

Thoughts?

 

[Christopher17]

This is one that nags at me, the 2 home brew 1/4 wave GP I built, one the radials is grounded to the mast, the other does not, both work good, both have separate ground wires with copper rod in the ground, I have made sure to make both the mast and ground wire a non resonate length, I do believe that it should be isolated from the mast, so far unable to prove to myself

 

[bob85]

IM no Expert,

NEC don't lie if you use it correctly,

I can't use NEC so I take a belt and braces approach & don't let masts gate crash the party by isolating them from the radial /coax junction with a Fiberglas isolator, some people have used spade or fork handles,

I want as little tx & rx from my mast & coax as practicable,
if you don't have a cmc problem you are unlikely to see any change by isolating it,

if you do have a problem then isolation does work & can lower local noise, eliminate rfi & improve your signals a little,

4x 1/4wave radials does reduce cmc, many people won't notice any cmc problems with 4 tuned radials,

but that does not mean you can't have cmc just becasue you have 4 radials if you pick an unlucky electrical length of either mast or coax braid,

DC is OK, its avoiding a low common mode impedance for rf that matters.

If you use NEC you can see what lengths cause higher cmc & take measures to avoid the situation.


Most of my buddys are not electrically minded but they understand mechanical stuff fairly well,
so i tell them imagine your antenna is a sealed drum been fed by a pump at 50psi 10gallons per hour ( your radio )

drill 4 holes in the side of the drum ( your radials ) so water squirts out under some pressure,
now drill 2 holes in the bottom of the drum ( your coax and mast)

the water from the holes in the side of the drum reduce flow because its also leaking from the bottom of the drum,

next shove a bung in one of the holes in the bottom ( you just isolated your mast )
now water squirts a little faster from the side holes and the remaining hole in the bottom,

now add a second bung with a small hole ( your choked coax ) in the remaining bottom hole so very little water squirts out & the side holes squirt harder, almost as hard as before you drilled two holes in the bottom,
that usually turns a light on in their head,

antennas are reciprocal so it works both ways tx & rx.

 

[davev8]

now it was a little time ago when i came to this conclusion so i hope i not forget i was modeling 5/8 wave with 4 radials using manma software (much easier than NEC) just playing with the length of the radiator till i had best gain at a low angle which ended up very slightly longer than 5/8 wave ..then i decided to put it on a mast ...the gain went up a little but the pattern went tits up from a nice and low take off angel to much more sky words .so i started to play with the length again and found that by shortening the radiator that ended up somewhere halfway between 1/2 wave and 5/8 wave i got the gain back at a low angel ...i also found if i added a mast but had a 12 inch gap between it and the antenna to represent an insulator its still affected the take off angel but not as much ..
my conclusion was that as we know the longest we can have a radiator is 5/8 ish wave before the pattern goes to pot i think the antenna is coupling to the mast and making it think its longer than it really is

 

[Rwb]

Got a question about whether you should or should not ground elevated omnidirectional antennas (that aren't designed to use the mast as part of the antenna).

Copying Bob85's signature:

W8JI
"An antenna with a poor ground using few radials cannot have a support mast grounded to the radial common point (at least it shouldn't if designed properly) There is no exception to this!

Does 4x 1/4 wave ground radial constitute a "poor" ground? The modeling that I have seen would seem to indicate this - i.e. isolating the antenna from the mast shows it to perform better.

What about DC Grounded antennas? If you use a RF Choke at the feed point to eliminate CMC on the braid, you still give DC a path to ground.

Thoughts?

Expert? EXPERT= DRIP UNDER EXTREME PRESSURE

 

[freecell]

Not enough radials, inadequate feedline decoupling. https://static.dxengineering.com/global/images/instructions/dxe-fcc050-h05-b.pdf

 

[freecell]

This is what happens when there aren't enough radials to decouple the feedline from the antenna. Radiator current looking for a return circuit leaks through the elevated radials to the random length "radials" represented by the conducting support structure, the outer shield of the feedline, conducting guy wires, earth ground beneath and any other conducting materials in the near field of the antenna. The radiation efficiency is severely restricted due to ground resistance losses preventing all power delivered to the load from being radiated.

Reference: There seems credible acceptance by some writers that the Ground Plane Antenna was invented by George Brown (more completely Brown, Lewis and Epstein (BLE)) from RCA, and is described in US Patent 2,234,333 for a Demountable Antenna filed in 1939.

 

[freecell]

https://www.worldwidedx.com/gallery/albums/elevated-radials-and-decoupling.761/

 

[The DB]

https://www.worldwidedx.com/gallery/albums/elevated-radials-and-decoupling.761/

Wow, just wow. Did you actually look at the models in that linked album?

I have a few suggestions for you.

Look at what is being shown in the album, specifically the two models at 36 feet above ground. I will post them here side by side to show you...

[photo=medium]6508[/photo][photo=medium]6510[/photo]

Does doubling the number of radials affect the currents on the mast at all? Nope.

Will adding more radials have any significant effect? Nope.

Why? Ohm's Law.

Next, you should look at what the person who created the album said on this matter in the other thread that you brought this up.


The DB

 

[TheBlaster]

For DX ? It is irrelevant it is pure luck of conditions. Ground / no ground will make no significance whatsoever. I suggest forget it and transmit. I am slowly concluding there is little point wasting time on minutia of wire antenna set up when it is 100pct conditions that determine your dx opportunities.

Your time is magnitudes better spent calling "CQ DX" or listening.

I am not an expert but I can become one slowly, the illusions are being dispelled through operating as my statement above suggests.

Any old rubbish or a 1/2 decent quality factory omni antenna will or will not make a contact on any given time or day. 5 meters of ATU tuned wire will work dx as will a coil on a stick 2m antenna whip.

You either go QRO and a 4 element beam on 23m mast or hope for the best. That is the 2 options you have a as a ham on HF. Spend on beams and max legal power or make your own DIY antennas.

Take your pick if you have the land and money that is. When you can make peace with these 2 options you can become a happy ham.

And if that is not enough just do as Italian's and Russian's and sh** all over the bands and other hams, over drive, run illegal power, over modulate, crackle and sound like Davros from Dr. Who.

Make conditions with beams and massive Watts or just be a chancer.

Currently I am trying to work out where skill enters ham radio so far skill is lost in the massive void of pure luck and who has the biggest Watts/Antenna.

As far as I gather on an elevated antenna 4 1/4 waves just about does the job of being an ok ground plane, ground mounted you need much more, 32 radials will do it.

Enjoy.

 

[TheBlaster]

Plug this into your ATU, load the coils... give it a few days and you will probably work the world.. and I am not even really joking... given conditions you would.