Ground plane vs metal mast

[Confused]

Hello, I've been lurking on this site for a couple years or so. This is my first post. I haven't been able to find the answers I've been looking for. Or maybe I did but they were over my head. So I'm hoping that someone here can dumb it down enough so that even I can understand it.
If you take a 1/4 wavength antenna, (at 11 meters) like a 102" mobile whip (108" times a velocity factor of approximately 95% equals about 102.5") and mount it on a 102" metal mast, you will have more or less created a 1/2 wave dipole (I think). If you then remove that whip and mount a 1/4 wave ground plane in it's place, it seems to me that you now have a COMBINATION of a 1/4 wave ground plane (50 ohms) AND a 1/2 wave dipole (73 ohms). The mast would still be a part of the antenna structure, right? Most antennas seem to be modeled in free space and don't include the mast. What effect does a big chunk of metal mast have on the properties of the antenna? Would the impedance be somewhere between 50 and 73 ohms? Would a different length mast that isn't resonant be worse than one that is resonant? Should the mast be isolated? Even if you isolate the mast but run a ground wire from the ground plane to a ground rod, wouldn't that also be a part of the ground plane assembly? I bought a 1/4 wave base antenna and I'm wondering if I should isolate the mast or cut it to a resonant length. I'm thinking I should cut the mast to 102 inches or so, but then I wouldn't have enough clearance between the radials and the roof. A guy from South Carolina on YouTube made a 1/2 wave vertical dipole from copper wire and PVC pipe and then added two ground plane radials made from wire and had a good match and could talk skip on it really well. What am I missing? What do I need to know? Thanks!!!

 

[Justme]

Half wave dipole = 73 Ohms.
Gp with radials 50 Ohm, if the radials are set at the right angle.
Yest the metal pipe will have an influence, even with 4 radials a GP won't completely find the ground plane 100% effective and all metal connected to it will work with it.

With 4 radials it won't be neccasary to cut the pipe to resonance, the influence wil be minimal.

 

[Tokin]

Welcome to the forum!

 

[bob85]

it is using the radials and the 1/4wave mast in parallel with the radials,
current will share between them depending on their common mode impedance,
1/4wave mast has a low impedance & high current,

that's OK so long as you isolate the mast & coax just below the radial tips, it will still act like a dipole or similar in performance,

the problems with that style antenna rear their ugly head when your mast is longer than 1/4wave & you don't isolate the mast & coax below the radial tips,

the mast & coax will carry current depending on their electrical length been grounded or not at the bottom end,

w8ji calls it picking a lucky length mast & feed-line, people get wildly different results from the same antennas, then blame the antenna,

the fact you are thinking/asking about the current in the mast is a great start
there does not need to be luck involved,

most of the difference in signals seen when comparing CB verticals is not the antenna itself, its the mast & coax carrying & receiving current that's messing the whole shebang up,

proper installation is the best equalizer of signals when comparing antennas that I know of.
proper installation is the best reducer of local noise picked up on coax & mast,
proper installation is NEVER how the manufacturer tells you to do it,

the last thing they want is their 1/4wave groundplane equaling or edging ahead of their top line 5/8wave at the same tip height.

 

[BJ radionut]

the last thing they want is their 1/4wave ground plane equaling or edging ahead of their top of the line 5/8th's wave at the same tip height.

 

[357magnum]

" Oh My Antenna God ( Marconi ) " , Speak your Piece " Stay Healthy & Safe ! 73 & God Bless ! Leo

 

[The DB]

Confused,if you need me to and no one else does, I can go though your questions one by one. But with the setup you are talking about I wouldn't worry about using a resonant mast length, or isolation, or other said anything unless you have a problem with said setup first.

Sure, there are antennas where the concerns you posted can be a big deal, your antenna is not one of them. In fact, the primary benefit of having 1/4 wavelength radials is they are very good at preventing the very problems you are concerned about.


The DB

 

[357magnum]

My apologies "Db" ! I forgot about you ! Still on my First Cup Of ! Stay Healthy & Safe ! 73 & God Bless ! Leo

 

[The DB]

My apologies "Db" ! I forgot about you !

What? You forgot about me? I'm hurt...


The DB

 

[Justme]

An 1/4 wave GP will have higher lobes as an 5/8 wave antena.
An 5/8 antenna its self does not add power it just sends rf in a lower lobe or bundles it better as an 1/4 wave which "can" be a benefit.
Sometimes an 1/4 wave GP just can have the edge over an 5/8 antenna depending on the higher lobes working better at that moment for you.

If in doubt put up the GP isolated, and fit an 1:1 choke directly under the plug at the coax, so that nothing can interfere with the radiation pattern, and just the antenna does the work.
My bet is that you won't notice muchy difference, we are talking 1 dB changes here where we can just make out 3 dB differences in signals.
My Imax 2000 is mounted on a 7 feet alu pole on the roof ( non resonant length) has 17 feet radial wires because it is used from 10 back to 17 meters on 5 bands, (10/11/12/15/17 meter)
With isolation and without no differences were noted, the 4 radial wires were enough.( clamp on ferrite at the connector type 31 material)
My Diamond X 510 N is mounted on polyester pole ( 2 meter and 70 cm) has 3 radials for it and clamp on ferrites around the coax, no difference with or without the clamps, meaning the coax is "clean" from RF.
Since the mast is polyester it will play no role.

Back to the original poster Q:
Put it up and see how it works, if it is an GP with 4 full size radials it will work well.
There are no miracle antenna's and you can go the short way set it up and don't worry, or put it up isolated from the mast and use a 1:1 choke at the feedpoint on the coax to isolate the whole antenna.

Problem will be that any metal within 1/2 wavelenth off the antenna will have some influence on the radiation pattern through coupling.
When i transmit on my 160 meter vertical i can measure the rf on my OCF even if that is horizontal and well away from the vertical,
Even isolated your mast will still have an influence or you should use an polyester mast, and still have to choke the coax.
Life isn't all that simple, theory and practice...

 

[bob85]

imho the biggest reason to isolate your antenna and mast is to minimise the noise that most anybody in an urban environment has to do battle with nowadays.

sherweng.com pfft
your ft101d ic7300 7600 ts590 won't hear what i can hear on a cb if you have more noise than me, its as simple as that,
i don't even need to use a good receiver to outhear locals who use some of the best receivers out there,

garbage noise on cbfor many people is WAY higher than the noise floor of even a crappy cb receiver,

you don't need the latest sherweng champion lowest noise best blocking highest dynamic range rig,

you just need less noise than the other guys to hear & talk to folk they can't hear on ANY radio,
i do it most days.

 

[Justme]

Lets see.
160 meter S6 noise on vertical, horizontal S4 (worked 6 japanese stations now)
80 meter horizontal S3 noise vertical s4/
40 meter s1 noise vertical, horizontal give it 1/2 S
20 an above 0 S points noise.
Seems i don't have much of a problem on the edge of the city.

 

[Confused]

it is using the radials and the 1/4wave mast in parallel with the radials,
current will share between them depending on their common mode impedance,
1/4wave mast has a low impedance & high current,

that's OK so long as you isolate the mast & coax just below the radial tips, it will still act like a dipole or similar in performance,

the problems with that style antenna rear their ugly head when your mast is longer than 1/4wave & you don't isolate the mast & coax below the radial tips,

the mast & coax will carry current depending on their electrical length been grounded or not at the bottom end,

w8ji calls it picking a lucky length mast & feed-line, people get wildly different results from the same antennas, then blame the antenna,

the fact you are thinking/asking about the current in the mast is a great start
there does not need to be luck involved,

most of the difference in signals seen when comparing CB verticals is not the antenna itself, its the mast & coax carrying & receiving current that's messing the whole shebang up,

proper installation is the best equalizer of signals when comparing antennas that I know of.
proper installation is the best reducer of local noise picked up on coax & mast,
proper installation is NEVER how the manufacturer tells you to do it,

the last thing they want is their 1/4wave groundplane equaling or edging ahead of their top line 5/8wave at the same tip height.

I'm trying to figure out what a proper installation is. Should I use a coil choke just to be on the safe side? Antenna voodoo just doesn't make any sense to me.

 

[Confused]

Welcome to the forum!

Thanks!

 

[Confused]

Confused,if you need me to and no one else does, I can go though your questions one by one. But with the setup you are talking about I wouldn't worry about using a resonant mast length, or isolation, or other said anything unless you have a problem with said setup first.

Sure, there are antennas where the concerns you posted can be a big deal, your antenna is not one of them. In fact, the primary benefit of having 1/4 wavelength radials is they are very good at preventing the very problems you are concerned about.


The DB

That sounds good to me, I just wish I could wrap my brain around this stuff.