It's sometimes spec'd because some people just don't know what they're doing, both on the buyers and sellers side.
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Checking duplexer
- Thread starter FreddyM
- Start date
FreddyM
Member
My next question is on my mobiles.
I checked the swr on the antennas with the vna and with the swr meter.
All of them were around 1.3 and the swr meter was showing very little reflected power, just a few watts.
Does SWR tell me the whole story as far as the antenna is concerned.
It seems we have a big variance in range on the mobiles.
We are using tram 1159 and a few maxrad mhb 5802 antennas.
Should the base be firmly grounded to the frame of the vehicle on those antennas?
Should I take an ohm meter and check that?
Should the antenna be on the cab versus beside the hood where its actually lower than trhe cab?
mmckenna
I ♥ Ø
Low SWR just tells you that there's little reflected power. It doesn't necessarily mean the antenna is working well, or radiating that RF well.
The metal under the antenna matters. That's the ground plane. Even though they may be "no ground plane" antennas, any metal under the antenna is going to impact performance.
Ideally, the antenna should be permanently mounted dead center of the truck roof. There should be, at minimum, 18" of metal in all directions under the antenna. Center of the roof, even on a regular cab truck will do that just fine.
Mounting the antenna off to one side will result in a lopsided ground plane and will make the antenna slightly directional (in both transmit -and- receive).
Mounting the antenna down on the fender will result in that lopsided ground plane. It can also result in some shielding from the cab.
Permanent mount NMO installed in a 3/4" diameter hole drilled in the dead center of the truck roof is the ideal installation. Cut coaxial cable to length and install the appropriate connector. Radio 12 volt DC power should come directly off the battery. Don't tap into existing wiring or cigarette lighter plugs. That's a good way to introduce noise into the radio via the power leads, and can impact performance.
The Tram antenna you mentioned earlier is a high gain antenna. That might look good on the surface, and may work well in some applications, but that extra gain comes with trade-offs. The gain is achieved by focusing more power at the horizon. That's great if you are out on the plains, and the truck is always perfectly level. Might be great in Saskatchewan, might not if you are in the middle of BC.
The Maxrad is a standard half wave, and while it has a bit of gain, it's not quite as extreme as the Tram.
Since you have two different types of mobile antennas, it might be worth getting a proper NMO mount installed in the center of the cab, and then comparing antennas. Likely you'll find one works better than others in certain situations. But that situation will change as the trucks move around. Your own experience will tell you what works better most of the time.
Sounds like you have a couple of things to address:
Duplexer tuning. -call a professional.
Appropriate coaxial cable. -easy to order the right stuff.
Increase repeater antenna height. Height is important, but I don't think you're going to see a huge amount of difference going from 30 feet to 70 feet.
Permanent install of mobile antennas. Always a good idea. No compromise mounts, no mag mounts, no fender mounts. Do it right the first time.
The metal under the antenna matters. That's the ground plane. Even though they may be "no ground plane" antennas, any metal under the antenna is going to impact performance.
Ideally, the antenna should be permanently mounted dead center of the truck roof. There should be, at minimum, 18" of metal in all directions under the antenna. Center of the roof, even on a regular cab truck will do that just fine.
Mounting the antenna off to one side will result in a lopsided ground plane and will make the antenna slightly directional (in both transmit -and- receive).
Mounting the antenna down on the fender will result in that lopsided ground plane. It can also result in some shielding from the cab.
Permanent mount NMO installed in a 3/4" diameter hole drilled in the dead center of the truck roof is the ideal installation. Cut coaxial cable to length and install the appropriate connector. Radio 12 volt DC power should come directly off the battery. Don't tap into existing wiring or cigarette lighter plugs. That's a good way to introduce noise into the radio via the power leads, and can impact performance.
The Tram antenna you mentioned earlier is a high gain antenna. That might look good on the surface, and may work well in some applications, but that extra gain comes with trade-offs. The gain is achieved by focusing more power at the horizon. That's great if you are out on the plains, and the truck is always perfectly level. Might be great in Saskatchewan, might not if you are in the middle of BC.
The Maxrad is a standard half wave, and while it has a bit of gain, it's not quite as extreme as the Tram.
Since you have two different types of mobile antennas, it might be worth getting a proper NMO mount installed in the center of the cab, and then comparing antennas. Likely you'll find one works better than others in certain situations. But that situation will change as the trucks move around. Your own experience will tell you what works better most of the time.
Sounds like you have a couple of things to address:
Duplexer tuning. -call a professional.
Appropriate coaxial cable. -easy to order the right stuff.
Increase repeater antenna height. Height is important, but I don't think you're going to see a huge amount of difference going from 30 feet to 70 feet.
Permanent install of mobile antennas. Always a good idea. No compromise mounts, no mag mounts, no fender mounts. Do it right the first time.
FreddyM
Member
FreddyM
Member
Does the amount of metal matter?
It seems to me bigger equipment(combines and semis) do a better job than smaller equipment( 3/4 ton trucks) especially in recieve mode.
mmckenna
I ♥ Ø
The half wave antennas do not -require- a ground plane. Sometimes installers will place a metal sheet/tape/etc. under the antenna to provide one. I had a UTV with a plastic roof. I placed a thin sheet of metal under it to provide the ground plane and help antenna performance. Not required with the half wave antenna, but it helped it perform a bit better.
mmckenna
I ♥ Ø
The amount of metal needs to be 1/4 wavelength radiating out from under the antenna equally in all directions to be ideal. 1/4 wavelength at your VHF frequency would be about 18 inches. More is OK, less isn't. That's why you want it mounted in the centre of the roof with an equal ground plane under it. A "perfect" ground plane for VHF would be a 36 inch diameter circle with the antenna mount right in the centre of the circle.
The combine benefits from having the antenna up a higher, which helps range.
kayn1n32008
ØÆSØ Say it, say 'ENCRYPTION'
Can you provide your call sign? There are those of us that know how to retrieve the license and then we can provide you with more information.
I'm guessing you purchased through Prairie Mobile?
Implicit in most of the advice above should be that significant skill/craftsmanship is required. Correctly installing RF connectors can require study and experience and varies by type. In a mobile install, cutting off the last 5 feet of a 17-foot cable and replacing the connector may be the wrong thing to do just to save 0.2 dB that will not be noticeable to anyone, perhaps at the risk of incorrectly installing a connector that can result in much more trouble. Similarly, replacing a working LMR-400 feed with hardline, requiring correct assembly of the connectors at each end, seals, arrester feed-through, jumpers, even more careful attention to bend radius, proper type and use of clamps, etc., could be the wrong choice, too. Definitely start with a field call by a qualified tech to tune/check the repeater and duplexer.
speedway_navigator
Member
In auto racing I used the aluminum foil tape used by HVAC folks to seal vent pipes.
dickie757
Wired
If duct tape is "200 mph tape," then that stuff is "Mach 2 tape." I have never used it as a counterpoise, but have used it to hold down my hood. It made checking the oil a real task.
speedway_navigator
Member
No, it only shows the amount of reflected power which includes coax loss which can mask a coax or antenna problem.
Using this adapter will show power going into the antenna, not just what the radio puts out.
Suppose your coax has an insertion loss problem resulting in a 3 dB / 50% loss. If you see 30 Watts out and 3 back at the radio you might think you have a 10% reflected power ratio or 10 dB return loss.
That equates to a 4 dB return loss at the antenna since it takes 6 Watts reflecting from the antenna to show 3 at the Wattmeter.
An Ohmmeter is only for pure resistance, 50 Ohms in the RF world is impedance, which has inductance and capacitance added into the resistance. A VNA, vector network analyzer, shows impedance in Ohms on a Smith chart.
FreddyM
Member
I wasn't clear with my question.
I was referring to how important it was to ground the base of the antenna to the frame of the vehicle.
FreddyM
Member
Thank's for the tip.
I was wondering where to source something like that.
FreddyM
Member
How do I know my antennas are radiating RF well?
mmckenna
I ♥ Ø
Field strength testing.
Comparing to manufacturer specs.
Comparing performance to realistic expectations based on the equipment/site used.
Experience.
speedway_navigator
Member
I tried explaining a ground plane to a race engineer once, when I came back to test the antenna he had a green ground wire from the NMO to the chassis. Too bad this was long before cell phone cameras.
The concept of a ground plane is described in lots of webpages and videos, I won't duplicate the effort. The metal from the antenna does not need to be conductive with the vehicle.
These plots show return loss for my mag mount dual band antenna, one with the base on the hood, then with a magazine insulating it from the hood. It's interesting to see how the return loss at VHF (marker 1) is double insulated from the hood as on the hood. The opposite happens at UHF, the loss goes from 34 to 48 dB (marker 2).
The worst case scenario is VHF on the hood, 17 dB return loss means reflected power is about 2% of what leaves the VNA.
The concept of a ground plane is described in lots of webpages and videos, I won't duplicate the effort. The metal from the antenna does not need to be conductive with the vehicle.
These plots show return loss for my mag mount dual band antenna, one with the base on the hood, then with a magazine insulating it from the hood. It's interesting to see how the return loss at VHF (marker 1) is double insulated from the hood as on the hood. The opposite happens at UHF, the loss goes from 34 to 48 dB (marker 2).
The worst case scenario is VHF on the hood, 17 dB return loss means reflected power is about 2% of what leaves the VNA.
Attachments
speedway_navigator
Member
Measuring with a spectrum analyzer like the double M guy said.
this is very affordable compared to what we have in our service monitors or stand alone units.
If you are testing a portable have a friend record the dBm level with you holding the radio in front of you, then with it on your belt like you would with a spkr mic. Test with the radio in the middle of a vehicle hood, trunk and roof to see how much more power you get from the ground plane effect.
If the spectrum analyzer is in front of the vehicle the hood and roof should be better than the trunk signal due to some blockage.
