I have a question for the more electronically adept members. I have a motorhome with a 3/8 x 24 base mount antenna on the roof. The whip is about 15" in length. On a spring mount. It is listed as a CB antenna. What I need is an antenna for a GMRS radio at 462 mhz. The question is, can I cut the whip to 12.125" which is close to the listed 1/2 wave length for a 462 mhz antenna, and expect reasonable results. I am going into Mexico on a 100+ day trip with a small caravan, and they use GMRS radio for intra caravan communications. I guess buying an SWR meter and looking at the result is the best answer, but I thought asking the C-Brats is a place to start.
Antenna tuning
- Thread starter Discovery
- Start date
Micahbigsur@msn.com
New member
SWR meter, start long and cut back bit by bit? But can't you get one of the rubber ones in 3/8s thred or an adapter? The off road race radio vendors have some cool stuff on line.
Good luck and safe travels.
Good luck and safe travels.
jkidd
Active member
Brent have you look at one of these
https://www.amazon.com/MXT115-Watt-Micr ... 191&sr=8-4
This one is good for 50 miles clear line of sight and they have them with more power than this. It comes with a mag base antenna.
On edit the gmrs frequency and the cb are a long way apart, might have to be longer (I haven't check) Seems like more work than it's worth. You can buy one for around $30.
https://www.amazon.com/MXT115-Watt-Micr ... 191&sr=8-4
This one is good for 50 miles clear line of sight and they have them with more power than this. It comes with a mag base antenna.
On edit the gmrs frequency and the cb are a long way apart, might have to be longer (I haven't check) Seems like more work than it's worth. You can buy one for around $30.
The problem with the 3/8 X 24 base is that there are no direct fit GMRS antennas. Everything GMRS has an NMO base. I can find NMO to 3/8 x 24 adapters, but not the reverse.
Jody, I have that exact radio antenna/combo. It's just that the motorhome has an antenna base with an already installed cable that fits the radio. Plus the motorhome is fiberglass and aluminum. I'd have to run the antenna cable out the window and glue a steel plate to the top of the motorhome for the magnetic base antenna.
I have tried the radio with the existing motorhome antenna, and I am getting some radio traffic. Commercial operators, electrical contractors, etc. I just don't know what frequency I'm listening to.
I don't have an SWR meter and don't know of one in our little town of 500 souls. Amazon Prime is only 4 days away. Yes, that's how rural we are.
Jody, I have that exact radio antenna/combo. It's just that the motorhome has an antenna base with an already installed cable that fits the radio. Plus the motorhome is fiberglass and aluminum. I'd have to run the antenna cable out the window and glue a steel plate to the top of the motorhome for the magnetic base antenna.
I have tried the radio with the existing motorhome antenna, and I am getting some radio traffic. Commercial operators, electrical contractors, etc. I just don't know what frequency I'm listening to.
I don't have an SWR meter and don't know of one in our little town of 500 souls. Amazon Prime is only 4 days away. Yes, that's how rural we are.
jkidd
Active member
Discovery":jz84cpyq said:
Plenty of swr's on amazon for around $20 but I think you might spend a lot of time and find out you can't make it match. I like your gluing the plate up there. I used a fender washer in the boat for a place for a mag base to stick to. Can you use the other base to bolt the plate to it?
I have held several Mexican reciprocal ham licenses. Mexico (and many other countries) have different laws--and are very sensitive about use of any frequency near a military installation or check point. I was witness to a boat which was impounded and all radios confiscated, plus a huge fine, when they transmitted on a ham VHF frequency near a military installation.
The following is a quote from QSL a major ham radio forum: I have added the bold print:
A 15" whip would suggest that the CB (11 meter band approximately 27 MHz. ) is base loaded. This changes the characteristics of the antenna dramatically. The GMRS is 70 CM band and is about 462 MHz. Also with a fiberglass/aluminum RV, unless that antenna on the roof, is tied into the aluminum near the antenna, it is a no ground plane antenna. 15" single whip does not really fit well with CB--unless it is base loaded. 102" or about 36" are the standard CB whips. The shorter CB are usually a wound wire antenna, in a fiberglass or plastic case.
You can start trimming if you wish--and might find a resonate frequency, but it will be hit and miss. If you were nearer, we could run an antenna analyzer on the antenna, and go from there....
I probably would fashion a metal bracket, attached to some aluminum part of the coach, and use the antenna with came with the Midland radio. This can be close to the driver's window--maybe even involving the mirror bracket. Even setting the Midland antenna inside the windshield on top of a steel pan (6" diameter for 462 MHz) would work--probably better than hacking up the CB antenna.
Another thing to think about is that coax cable loss is high in the UHF range. You really would want a high quality (big) UHF cable for max effective radiated power (ERP); something like the Belden 9913.
I have been running ham rigs from RV's for about 40 years, and usually put the antennas on the ladder, which I made sure was well screwed into the metal frame--in some cases, I used straps to the chassis. I used one of the better coax like the Belden 9913, and ran it under the coach, and up the side of the ladder. This was both HF and 2 meters--didn't use 70 cm.
One other thought that is 70 cm is very close to 3rd harmonic of 2 meters, so 2 meter loaded whips, tuned may work well on the GMRS.
The following is a quote from QSL a major ham radio forum: I have added the bold print:
A 15" whip would suggest that the CB (11 meter band approximately 27 MHz. ) is base loaded. This changes the characteristics of the antenna dramatically. The GMRS is 70 CM band and is about 462 MHz. Also with a fiberglass/aluminum RV, unless that antenna on the roof, is tied into the aluminum near the antenna, it is a no ground plane antenna. 15" single whip does not really fit well with CB--unless it is base loaded. 102" or about 36" are the standard CB whips. The shorter CB are usually a wound wire antenna, in a fiberglass or plastic case.
You can start trimming if you wish--and might find a resonate frequency, but it will be hit and miss. If you were nearer, we could run an antenna analyzer on the antenna, and go from there....
I probably would fashion a metal bracket, attached to some aluminum part of the coach, and use the antenna with came with the Midland radio. This can be close to the driver's window--maybe even involving the mirror bracket. Even setting the Midland antenna inside the windshield on top of a steel pan (6" diameter for 462 MHz) would work--probably better than hacking up the CB antenna.
Another thing to think about is that coax cable loss is high in the UHF range. You really would want a high quality (big) UHF cable for max effective radiated power (ERP); something like the Belden 9913.
I have been running ham rigs from RV's for about 40 years, and usually put the antennas on the ladder, which I made sure was well screwed into the metal frame--in some cases, I used straps to the chassis. I used one of the better coax like the Belden 9913, and ran it under the coach, and up the side of the ladder. This was both HF and 2 meters--didn't use 70 cm.
One other thought that is 70 cm is very close to 3rd harmonic of 2 meters, so 2 meter loaded whips, tuned may work well on the GMRS.
Bob and Jody,
I appreciate your advice. I will try the included Midland antenna. It's only 5" high, so 1/4 wave about 2db. There is a 6 db antenna available on a magnetic NMO base, but it's 27" tall and I don't think I can get under many of the bridges I'll encounter if it's on top of the motorhome. Being magnetic I guess it would tip over, or maybe break the antenna. The 2 db antenna being mounted 12' high should do the job. I tend to overthink the situation. We will be in a 13 vehicle caravan and I don't know how far out the vehicles will get separated. We will have a rotating Green Angle escort contracted to be with us almost continuously over the 5400 miles route. I hope they will discourage unnecessary police contact.
This is a 100+ day trip departing Mission, Texas on January 4th, 2020. The trip of a lifetime. We have traveled in Baja California in our own vehicles for thousands of miles, since 1972 and have never had any bad experiences. Well, two flats at the same time near Loreto comes to mind, but the Green Angles hauled me 50 miles and back to get replacements. I hope our mainland trip can go as well.
I appreciate your advice. I will try the included Midland antenna. It's only 5" high, so 1/4 wave about 2db. There is a 6 db antenna available on a magnetic NMO base, but it's 27" tall and I don't think I can get under many of the bridges I'll encounter if it's on top of the motorhome. Being magnetic I guess it would tip over, or maybe break the antenna. The 2 db antenna being mounted 12' high should do the job. I tend to overthink the situation. We will be in a 13 vehicle caravan and I don't know how far out the vehicles will get separated. We will have a rotating Green Angle escort contracted to be with us almost continuously over the 5400 miles route. I hope they will discourage unnecessary police contact.
This is a 100+ day trip departing Mission, Texas on January 4th, 2020. The trip of a lifetime. We have traveled in Baja California in our own vehicles for thousands of miles, since 1972 and have never had any bad experiences. Well, two flats at the same time near Loreto comes to mind, but the Green Angles hauled me 50 miles and back to get replacements. I hope our mainland trip can go as well.
Great plan. Marie asked about an escort. I thought you would have one--the Green Angels are fantastic. I agree that it will be a great trip. Is this the "Caravanas de Mexico Yucatan"/Missions/all Mexico trip? We are jealous!
We have seen a portion of what they offer, as we have either rented cars or taken our motor bikes inland, during our several boating trips along of all of the Mexican Coast, except the Bay of Campeche.
You probably have a good list of spares, but I would take a couple of off the rim tires, and some extra fuel filters along with the "usual" spares. Start with a new engine air filter.
The nearest I have had for an antenna similar to what you are using was a "Wilson cell phone amplifier" antenna. It has the 3/8 24 mount, and I put a spring mount on the side of the RV, with a fiberglass 24" extension which brought it just above the roof line of the RV. It had tuned radial bars. Eventually it succumbed to a tree limb. I ran the cable down inside just aft where the RV forward window curtains were, This was "out of site" and cosmetically was acceptable. The coax went out thru the side wall and I used a marine fitting to be sure it was sealed on the exterior
Another option you have is the Midland MXTA6 Antenna Mounts which will work with the 6 dB Midland antenna. That can be mirror (depending) or flat mounted to the side wall of the RV, to get the antenna up higher--and using the technique outlined above. I have screwed the 4 bolt holes to the side of a fiberglass or aluminum RV for ham radios, using 5200 as a sealant. Worked very well.
462MHz 5/8ths wavelength 15-3/16 inch. The 5/8 Wavelength works very well and is 4 dB gain over a dipole.
We have seen a portion of what they offer, as we have either rented cars or taken our motor bikes inland, during our several boating trips along of all of the Mexican Coast, except the Bay of Campeche.
You probably have a good list of spares, but I would take a couple of off the rim tires, and some extra fuel filters along with the "usual" spares. Start with a new engine air filter.
The nearest I have had for an antenna similar to what you are using was a "Wilson cell phone amplifier" antenna. It has the 3/8 24 mount, and I put a spring mount on the side of the RV, with a fiberglass 24" extension which brought it just above the roof line of the RV. It had tuned radial bars. Eventually it succumbed to a tree limb. I ran the cable down inside just aft where the RV forward window curtains were, This was "out of site" and cosmetically was acceptable. The coax went out thru the side wall and I used a marine fitting to be sure it was sealed on the exterior
Another option you have is the Midland MXTA6 Antenna Mounts which will work with the 6 dB Midland antenna. That can be mirror (depending) or flat mounted to the side wall of the RV, to get the antenna up higher--and using the technique outlined above. I have screwed the 4 bolt holes to the side of a fiberglass or aluminum RV for ham radios, using 5200 as a sealant. Worked very well.
462MHz 5/8ths wavelength 15-3/16 inch. The 5/8 Wavelength works very well and is 4 dB gain over a dipole.
thataway":11xbqdrp said:
Yes, it's the "Caravanas de Mexico Yucatan" trip and we are excited and also a bit nervous. Our kids think we are crazy, but at 73, we can't let the grass grow beneath our feet too long. I just had the motorhome serviced and bought spare filters and a serpentine belt. The Tiffin RED 33AA takes regular 22.5" tires and they are available in Mexico. Our tires have 18,000 miles on them and look like new. DEF is the major concern in Mexico. I'll just have to leave full and haul 15 gallons with me. It's expensive and hard to find in Mexico. There are 6 class A's on the trip, so we all may be in the same situation.
I decided to just buy a Midland MXTA12 base and an MXTA10 3db antenna and put the CB antenna back on the 3/8 x 24 base. Maybe I'll use a CB sometime in the future.
Micahbigsur@msn.com
New member
Bob, as too the Mexican law on these radios, the Baja racers have headset multiple channel inter vehicle, inter team systems, they never mentioned problems?
Thanks for reminding us to be careful of what frequencies we are using in Mexico as our 706 III G is set up for most.
Thanks for reminding us to be careful of what frequencies we are using in Mexico as our 706 III G is set up for most.