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gmrs ground plane omni antenna


motsco
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howdy all, 

yes another newb, 

can i mount a omni directional antenna to a ground plane?

antenna is a tram br-6140

the ground plane is a tram 1465

i have the connectors to make them mate.  

good  idea or bad idea?

i have a 1000g in my truck.

as of now for home base i’m using a 935g through 50 feet of 8 u to a nagoya 770g on the 1465 mount  

i can hit the 2 closest repeaters on 5w    just trying to improve until i can afford another 50w radio  

thanks 

tom .

 

 

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6 minutes ago, motsco said:

i moved it down to where the antenna metal was about inch over the pole and tx and rx got a lot better on repeaters

If I read you correctly, in both cases the top of your mast was below the yellow/brass metal part of the antenna? Good then.  What's likely happened is a bad contact/connector got better with the handling of the antenna.

At any rate, use the configuration that works better. Antenna tuning is considered a black art by many. It is not, of course, but as long as a setup works for you it's good. You can always debug it later.

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27 minutes ago, motsco said:

howdy all, 

yes another newb, 

can i mount a omni directional antenna to a ground plane?

antenna is a tram br-6140

the ground plane is a tram 1465

i have the connectors to make them mate.  

good  idea or bad idea?

i have a 1000g in my truck.

as of now for home base i’m using a 935g through 50 feet of 8 u to a nagoya 770g on the 1465 mount  

i can hit the 2 closest repeaters on 5w    just trying to improve until i can afford another 50w radio  

thanks 

tom .

 

 

The BR-6140 is a half-wave dipole, having ~2.15dBi (0dBd) gain. Mounting closely over a ground plane is going to distort the radiation pattern (probably sending most of the energy up into the sky rather than horizontally).

Generic (hence the 300MHz) vertical half-wave dipole in free-space:

image.png.7caac16beb3a765013ced3a7fd3a665b.png

Oh, you'll want to compare the 3dB beam width too... The 3dB points are where the power is half of the maximum.

Same antenna located 1/4 wave above ground:

image.png.4fd6e739e17e0cc9d3a730be6f06a1cf.png

Strongest signal is on a 14 degree take-off angle -- might be good if you are only trying to reach repeaters on tall mountains. Note the reduction in gain, however -- 1.89dBi

Located 0.01 wave above ground (the NEC2 software core doesn't like wires touching actual ground, and I can't justify paying LLNL for a license to NEC4 or NEC5):

image.png.51613e8f3a1a37b751b79d9179ec67b3.png

Smoother lobe, but a 19deg take-off angle. But only 0.79dBi gain!

Mounted 20 wavelengths up:

image.png.b4644a65c1175ed64fdfc6f7f6fec8f3.png

Take-off angle is only 2deg, practically horizontal -- and with ground effects nearly 7dBi gain

 

The Nagoya requires a ground plane, and using just raw dimensions is 1.5 wavelengths long. I'm not sure if it is base-loaded, center-loaded, or both (I'm used to antenna with a central "lump" being center-loaded -- better radiation pattern, but also more wind drag). Or possibly the center lump is a phasing system and the antenna acts as two phased stacked verticals -- a 5/8 wave is 0.625 wavelengths long, so two stacked 5/8 waves could be about 1.5 waves long.

NOTE: the Nagoya is a 6dBi antenna. That's twice what the half-wave dipole has for gain unless the half-wave is mounted, as shown, 20 waves above ground (actually, with the bottom just one wavelength above ground, the half-wave shows around 6dBi, but with a higher take-off angle). Presuming it is mounted on a proper ground-plane, it should outperform the half-wave mounted close to ground. At one wavelength above ground, the half-wave will start to compete with it.

 

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Try both. For receive, they should be the same. For transmit, I guess 6140 will outperform Nagoya for a reason that your ground plane (4 radials) needs to be tuned for the 462MHz. It is not a UHF groundplane in it's stock form. 6140 is as fool-proof as it gets, while tuning Nagoya+groundplane may give you better results eventually. Or may not.

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If planning a base station, you really want to get the antenna above the roof line. Using an artificial ground-plane (by this I mean one not built into the antenna itself) just feels like adding complications.

Fitting the half-wave at the top of a 6ft pole does put it a few wavelengths above real ground (wavelength for 465MHz [splitting simplex and repeater]) is ~65cm (2.1ft). Call it three wavelengths up from real ground (for purposes of the graph, I had EZ-NEC rescale the generic to 465MHz, and set the bottom at a height of 2m (a bit over 6ft)

image.png.4c9fcb4d370051dfe31e1c0eb06fc63b.png

Maximum gain is 6.03dBi at a takeoff angle of 4deg. However, with such a low (relative to buildings) height, you are sending your energy through all the buildings between you and target (at least, until the 4deg rise brings you above neighboring buildings). Also, with a 50W transmitter, and the antenna just above head height, you may run into RF Exposure problems. I don't know how long a coax you are planning to run, nor what type: assumption -- 20ft Belden 9913... Coax loss drops power at the antenna to 44W, assume you talk one minute out of every three; for effective purposes, ignore the distance to area of interest -- look at the compliance distance figures. You, as "knowing" operator need to stay 3 feet from the nearest part of the antenna to meet FCC exposure criteria. Visitors, who don't know about RF exposure concerns, have to be kept 6.5 feet from the nearest part of the antenna. (For the real nasty version: https://www.govinfo.gov/content/pkg/FR-2020-04-01/pdf/2020-02745.pdf )

Evaluation for: GMRS vertical half-wave at 6ft

Input Parameters
Power to Antenna (from Exemption Worksheet) 43.95 Watts
TX Time Ratio (TX / (TX + RX)) 0.330  
Antenna Gain in dBi (dBd + 2.15) 6.03 dBi
Operating Frequency 465.000 MHz
Ground Reflection Included True  
Distance to Area of Interest 10.00 Feet
Maximum Permissible Exposure (MPE)
Controlled Environment 1.550 mW/cm^2
Uncontrolled Environment 0.310 mW/cm^2
 

Operating Mode/Factor: FM/AM/RTTY/PSK-31 (100%)

Average Power at the Antenna 14.504 Watts
Estimated Power Density at AoI Distance 0.12749 mW/cm^2
 
  Compliance
Distance
AoI in
Compliance
Controlled Environment 2.87 feet True
Uncontrolled Environment 6.41 feet True

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here’s my 770g now going through 50’ of rg8u  to 5w ht. i can hit simplex at about 4 miles all around , i can talk on repeaters from 5 miles and one 25 miles to my southwest. the ones north east that are closer 13 to 16 miles i can only get tones from when it’s dark out the radio in my truck can hear them and talk. but they go in and out. 

F3B821D0-9ACE-4604-A697-D74BC8AF5429.jpeg

676E6AD2-0EA9-47D6-B26E-BCBAD152F172.jpeg

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15 hours ago, motsco said:

here’s my 770g now going through 50’ of rg8u  to 5w ht.

Per https://www.timesmicrowave.com/Calculator?Product=RG-8/U&RunLength=50&Frequency=467 you'd have 2.3dB of loss for that cable at GMRS frequencies. If the HT is putting out the rated 5W, the antenna is only seeing 3.8W.

LMR-400 https://www.timesmicrowave.com/Calculator?Product=LMR-400&RunLength=50&Frequency=467 would have 1.4dB loss => 4.25W at antenna

Is it enough to make a difference WRT your antenna? I can't say. You state reaching some repeaters, but not others, but without a map showing antenna, nearby buildings, etc. I can't guess at what may interfere. All I /can/ state is that that antenna is too low for my comfort, with too much clutter nearby (even though that hanger/bird nest/feeder is below the ground radials of the antenna, if it is wrought iron it /may/ affect the pattern, adding some directionality to your antenna; this also applies to mobile installations -- the strongest signal tends to extend in the direction /away/ from the most metal).

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16 hours ago, motsco said:

here’s my 770g now going through 50’ of rg8u  to 5w ht. i can hit simplex at about 4 miles all around , i can talk on repeaters from 5 miles and one 25 miles to my southwest. the ones north east that are closer 13 to 16 miles i can only get tones from when it’s dark out the radio in my truck can hear them and talk. but they go in and out. 

F3B821D0-9ACE-4604-A697-D74BC8AF5429.jpeg

676E6AD2-0EA9-47D6-B26E-BCBAD152F172.jpeg

Getting the antenna up higher will make a big difference. Try to get the antenna to at least clear the roof top of your house, and any near by if possible.

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next question when mounting the 6140 to the pole. where does the metal at the bottom of the fiberglass antenna  need to align with the metal pole?

metal on antenna even with the top of the pole or under the top of the pole?

thanks,

tom 

 

 

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33 minutes ago, motsco said:

when mounting the 6140 to the pole. where does the metal at the bottom of the fiberglass antenna  need to align with the metal pole?

No, no need to align. Use the mounting brackets that should be supplied in the package with the 6140. Radiating element of this antenna is inside the white plastic tube, so make sure your mast does not block any part of it. In other words, mount 6140 on the top of the mast. As long as mast does not block the actual antenna (white plastic tube) it does not matter if it is aligned with the mast or offset.

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30 minutes ago, motsco said:

next question when mounting the 6140 to the pole. where does the metal at the bottom of the fiberglass antenna  need to align with the metal pole?

I'd put it no lower than the top of the pole. The main purpose for the lower metal section is, I believe, to withstand the strain of the mounting brackets -- clamping directly to fiberglass is likely to crush the fiberglass (and if it doesn't, a strong side wind might; metal at the top is  likely to prevent cracking of the fiberglass, and seal the internals from weather).

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10 minutes ago, axorlov said:

No, no need to align. Use the mounting brackets that should be supplied in the package with the 6140. Radiating element of this antenna is inside the white plastic tube, so make sure your mast does not block any part of it. In other words, mount 6140 on the top of the mast. As long as mast does not block the actual antenna (white plastic tube) it does not matter if it is aligned with the mast or offset.

 

7 minutes ago, KAF6045 said:

I'd put it no lower than the top of the pole. The main purpose for the lower metal section is, I believe, to withstand the strain of the mounting brackets -- clamping directly to fiberglass is likely to crush the fiberglass (and if it doesn't, a strong side wind might; metal at the top is  likely to prevent cracking of the fiberglass, and seal the internals from weather).

thanks, 

when i first mounted it i put it as tall as possible on the pole and on 2 repeaters i was was getting very low feed back on the 935 bars,  i moved it down to where the antenna metal was about inch over the pole and tx and rx got a lot better on repeaters,  haven’t found anyone to test on simplex yet. and haven’t tested swr   as of now i’d say the 6140 is better then the 770g.   

i’ll keep asking questions and testing 

 

thanks for the help. 

tom

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13 hours ago, motsco said:

when i first mounted it i put it as tall as possible on the pole and on 2 repeaters i was was getting very low feed back on the 935 bars,  i moved it down to where the antenna metal was about inch over the pole and tx and rx got a lot better on repeaters,  haven’t found anyone to test on simplex yet. and haven’t tested swr   as of now i’d say the 6140 is better then the 770g.  

If you had the antenna adjacent to metal (which I'd expect would kill the SWR) it may have acted as a director -- turning the omni into a directional antenna by reflecting some of the signal from the pole out the other side.

Or, being lower to real ground (whether the antenna is a ground-plane or not, you will have interactions with real ground) the take-off angle has been shifted upwards such that it now intersects the repeaters (but will have worse simplex operations as the beam "goes over" the heads of simplex users).

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29 minutes ago, motsco said:

i have access to get at tram 1486 new in box for 50$. would i be better off just using that?

Probably, as it will do the job just fine - the 1486 is what I have on my roof.

Now stand-by for all the long-winded and overly-complicated answers from the people that are incapable of grasping the meaning of "yes another newb" that you mentioned in your original question..

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28 minutes ago, OffRoaderX said:

Now stand-by for all the long-winded and overly-complicated answers from the people that are incapable of grasping the meaning of "yes another newb" that you mentioned in your original question..

What a fantastic fairy idiot! Look everybody, magic outside of Hogwarts.

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