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Comet SBB-7NMO Antenna - A Modification for GMRS


FarleyInglis
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At 12 miles out (level ground; the West TX Plains) to-and-from another SBB-7NMO antenna mounted 25ft in the air at my house, ... I made a Real-World Performance Test between the two antennas ... AND ... My OPINION is that the SBB-7NMO (without any modifications) performs better than the CA-2x4SR.

I was using 5 Watts Transmission from a BTECH-GMRS-V1 and got clear transmits and receives ... The CA-2X4SR transmitted with some-static that the SBB-7NMO didn't have ... (Note: This test was made 3 times by switching-out both antennas to verify this difference).

 

So, I decided to check the SWR on the SBB-7NMO antenna sitting in the middle of the roof of a 1999 2-Door Tahoe ... the results are as follows using the Low Power Setting with a BTech UV-50X2 ... (NOTE: BTech Specifications say 10watts; I'm showing 20watts on my SureCom SW102 Digital SWR Meter):

-----------------------------
SBB-7NMO -    No Modifications
-----------------------------
462.5500    GMRS        15        1.40
462.7250    GMRS        22        1.42
467.5500    REPT        15        2.00
467.7250    REPT        22        2.01

151.8200    MURS        01        2.09
151.8800    MURS        02        2.11
151.9400    MURS        03        2.13
154.5700    MURS BL     04        3.71
154.6000    MURS GR     05        3.73

146.5200    2M                    1.10
446.0000    70CM                  1.00
-----------------------------

I contacted a Comet Representative about what needed to be done to tune the SBB-7 Antenna for Dedicated GMRS use ... His reply was the following:

 

You can’t re-tune the SBB-7NMO to work in the GMRS bands.
 
There is a coil in the base of the antenna that is tuned for the impedance provided for the rod lengths as is.
 
Changing all three lengths of the rod sections can’t overcome the power feeding coils tuned frequency.

 

I have 4 of these antennas and decided to take a chance of destroying one to find out IF some modificaton(s) would improve the SWRs.  I found the following adjustments (or cuts) accomplished My Goal ... as follows:
--------------------------------------------------------
SBB-7NMO -    Rods Cut Lower-3/4" Middle-3/8" Upper-3/8"
--------------------------------------------------------
462.5500    GMRS        15        1.00
462.7250    GMRS        22        1.00
467.5500    REPT        15        1.15
467.7250    REPT        22        1.15

151.8200    MURS        01        1.45
151.8800    MURS        02        1.51
151.9400    MURS        03        1.54
154.5700    MURS BL     04        2.96
154.6000    MURS GR     05        2.99

146.5200    2M                    1.54
446.0000    70CM                  1.09
--------------------------------------------------------

-

In conclusion, I have shopped around for quite some time looking for a High-Gain GMRS Mobile Antenna ... but ... without finding anything to my satisfaction ... BUT ... with THIS Modification, My Quest has Finally Ended !!!

 

The GMRS Transceiver that I will be dedicating to this modified Comet SBB-7NMO Antenna is a BTech GMRS-50X1 which has 3 power settings 50 Watts, 20 Watts, and 5 Watts.

 

Just for completeness, the Mag Mount that I am using with all of my antennas is the "TRAM 1267R 5 1/2" Magnet NMO Mounting with Rubber Boot" that I have purchased from Amazon.

Happy GMRS'ing !!!,

 

FWI

.
 
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I have tested several of my 2M/70cm ham antennas on my KG-805G and they all have a very acceptable SWR.

456MHz is only about a 7% delta in frequency from 70cm (440MHz) so not a big surprise.

 

Keep this in mind though -  Probably no radio has an exact 50 ohm output.

Radios are designed to work well into a nominal 50 ohm load - but that does not mean the radio output is an exact 50 ohms.

Most radios are reasonably close to 50 ohms but not at all exact.

 

When I connected my 805G to my Arrow 7 element beam made for 70cm ham band the SWR was like perfect.

The reflected power needle didn't even budge.

Then I looked at the forward power needle.

The radio's normal output is around 4-5W but with the Arrow beam the forward power was more like 7W! 

Could this be possible?  Yes.

 

The Arrow beam probably just happened to be a reciprocal match for my 805G.

(And the length of coax probably was part of that equation.)

The 805G output power is spec'd into a 50 ohm dummy load which may or may not be the ideal match for that radio.

But this is how things are specified and tested.

 

Bottom line:

I think most 70cm ham antennas will work well with most GMRS radios but everyone needs to test their specific setup.

The length of coax may have a significant effect when the antenna is not an exact match.

 

Vince

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I have tested several of my 2M/70cm ham antennas on my KG-805G and they all have a very acceptable SWR.

456MHz is only about a 7% delta in frequency from 70cm (440MHz) so not a big surprise.

 

Keep this in mind though -  Probably no radio has an exact 50 ohm output.

Radios are designed to work well into a nominal 50 ohm load - but that does not mean the radio output is an exact 50 ohms.

Most radios are reasonably close to 50 ohms but not at all exact.

 

When I connected my 805G to my Arrow 7 element beam made for 70cm ham band the SWR was like perfect.

The reflected power needle didn't even budge.

Then I looked at the forward power needle.

The radio's normal output is around 4-5W but with the Arrow beam the forward power was more like 7W! 

Could this be possible?  Yes.

 

The Arrow beam probably just happened to be a reciprocal match for my 805G.

(And the length of coax probably was part of that equation.)

The 805G output power is spec'd into a 50 ohm dummy load which may or may not be the ideal match for that radio.

But this is how things are specified and tested.

 

Bottom line:

I think most 70cm ham antennas will work well with most GMRS radios but everyone needs to test their specific setup.

The length of coax may have a significant effect when the antenna is not an exact match.

 

Vince

 

Perhaps the following additional "measured Watts" Test-Information that I collected, but didn't originally report, MIGHT BE CONNECTED with what you are saying/describing.

 

Equipment/Environment Details:

 

BTech UV-50X2 at Low Power 20 Watts --- (NOTE: BTech Specifications say 10watts for Low Power; I'm showing 20watts on my SureCom SW102 Digital SWR Meter).

SureCom SW102 Digital SWR Meter displaying Frequencies, measured SWR, and measured Watts.

Two Comet SBB-7NMO Antennas both/each mounted on a TRAM 1267R 5 1/2" Mag Mount and each individually swapped-out and centered on the roof of 1999 4x4 2-Door Chevy Tahoe.

-----------------------------
SBB-7NMO -    No Modifications
-----------------------------
462.5500    GMRS        15        1.40   25.2W
462.7250    GMRS        22        1.42   24.7W
467.5500    REPT        15        2.00   16.8W
467.7250    REPT        22        2.01   16.6W

151.8200    MURS        01        2.09   20.9W
151.8800    MURS        02        2.11   21.3W
151.9400    MURS        03        2.13   21.6W
154.5700    MURS BL     04        3.71   25.8W
154.6000    MURS GR     05        3.73   25.7W

146.5200    2M                    1.10   22.3W
446.0000    70CM                  1.00   27.2W
-----------------------------
--------------------------------------------------------
SBB-7NMO -    Rods Cut Lower-3/4" Middle-3/8" Upper-3/8"
--------------------------------------------------------
462.5500    GMRS        15        1.00   25.4W
462.7250    GMRS        22        1.00   25.4W
467.5500    REPT        15        1.15   20.4W
467.7250    REPT        22        1.15   20.0W

151.8200    MURS        01        1.45   19.7W
151.8800    MURS        02        1.51   19.9W
151.9400    MURS        03        1.54   20.1W
154.5700    MURS BL     04        2.96   27.9W
154.6000    MURS GR     05        2.99   28.0W

146.5200    2M                    1.54   24.6W
446.0000    70CM                  1.09   26.7W
--------------------------------------------------------
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  • 3 months later...

Thanks for sharing this. I was considering the CA-2x4SR before reading this post but looks like a modded SBB-7NMO would be better for GMRS and MURS. I'm curious how you determined the appropriate cut lengths? Did you try other lengths before arriving at 3/4",3/8",&3/8" ? Are you still happy with it? Any problems with your radio? I'm wandering if slightly different cuts would wrangle in those MURS numbers without hurting the GMRS numbers much.

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