CB Antenna question

[homerjay]

Hello all. I have a question about my cb Antenna setup.....

From Antenna forward i have a fibreglass Antenna off of a freightliner truck mounted on my truck box lip behind the cab on a clamp on style bracket (made by myself to avoid drilling in my box) with your standard stud.

Feeding the radio is some RG59 picked up at my local radio shack. Radio is a cobra 29ltd classic.

My question is when I go to calibrate my antenna I get the same swr reading on CH 1 and 40. It is around the CAL mark on the scale.

Other antennas I have are a 2 ft CB fiberglass whip from a Mack (was broken from a 4ft that I shortened to use for my scanner) gives a reading of about 3 on 1 and r40 and a non tuneable fibreglass Antenna which gives a reading of about 2.5 on 1 and just a little higher the 2.5 in 40.

Is it safe to say I have an insufficient ground plane or is it something else?

 

[Blackswan73]

RG 59 is the wrong coax. You need RG 58, or 8x. RG 59 is 75 ohm. You need 50 ohm coax

 

[Project25_MASTR]

RG 59 is the wrong coax. You need RG 58, or 8x. RG 59 is 75 ohm. You need 50 ohm coax

Unless co-phasing. In that case two lengths of RG59 with a length of an odd quarter wave are needed and then a simple T with one end going to each antenna. RG58 (or other 50 ohm cable).


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[kf4eyr]

wrong coax,,, and how close to cab is antenna?

 

[jassing]

are you using 1 antenna off the truck and the truck had 2? These were probably cophased system and on big rigs, were usually no-ground due to all the fiberglass...that's what it sounds like, especially since you're using 75ohm coax...

 

[homerjay]

wrong coax,,, and how close to cab is antenna?

About an inch. Will post a picture when I get a chance

are you using 1 antenna off the truck and the truck had 2? These were probably cophased system and on big rigs, were usually no-ground due to all the fiberglass...that's what it sounds like, especially since you're using 75ohm coax...

I hadn't thought about that thank you.

 

[homerjay]

RG 59 is the wrong coax. You need RG 58, or 8x. RG 59 is 75 ohm. You need 50 ohm coax

My bad it actually RG58U I have

 

[kf4eyr]

inch from cab with a fiberglass antenna is a nono,,, need to get it away from cab and up above cab,,,, that would make your swr show bad being that close,,,,

 

[Project25_MASTR]

inch from cab with a fiberglass antenna is a nono,,, need to get it away from cab and up above cab,,,, that would make your swr show bad being that close,,,,

Sounds like the OEM install location. Freight does that on their more modern combo cabs as do a few other manufacturers.


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[homerjay]

inch from cab with a fiberglass antenna is a nono,,, need to get it away from cab and up above cab,,,, that would make your swr show bad being that close,,,,

Here's what i have. It is actually 3 inches from the cab.

 

[kf4eyr]

too close to cab would make swr go up,,, antenna too short,, needs more of it above cab,,,need to get it a least a foot from cab and a top or middle loaded antenna would be better,,,, i run a 102 inch steel whip on a 6 inch spring with a ball mount on the middle of my tool box,,,, works perfect for me,,,

 

[homerjay]

Thanks for all you help and input.

 

[JayMojave]

Hello homerjay: Great looking bracket good going. eyr is right the antenna is too close to the cab, causing a high SWR.

If that truck has a Break Light at the top middle of the can you can make a "L" bracket to go between the light and cab, and that will work much better. Good luck, let us know how you make out there.

Jay in the Great Mojave Desert

 

[Project25_MASTR]

Hello homerjay: Great looking bracket good going. eyr is right the antenna is too close to the cab, causing a high SWR.

If that truck has a Break Light at the top middle of the can you can make a "L" bracket to go between the light and cab, and that will work much better. Good luck, let us know how you make out there.

Jay in the Great Mojave Desert

It's a 2004-2008 F150 from the looks of it...so it should have the high mount brake light.

I'd personally be of the drill mentality but I wasn't like that until I got into FM gear. When I was driving a 2002 F150 I had my CB antenna mounted on the headache rack (which the OP doesn't have).

A longer antenna (4') might solve the issue but some more spacing and a longer antenna would most likely be the required solution. OP may consider a hood channel mount, but those don't seem to be very popular with pickup drivers for some reason.

 

[homerjay]

Close my truck is a 2010. It has the high brake like but I was avoiding a mag mount on the roof and avoiding drilling.

I was trying to mimic some that I've seen by doing a Google of stakehole mounts and I saw a few similar to that on trucks.

My guess is that they have a high swr as well.

Would a "No ground plane kit" from Firestik be the answer with their re tuned ngp antennas and different coax for ngp kits?

I was also hoping for a lower antenna. Such as in the pic it's 24 inches bUT I'm not objecting to a 36.
Thanks

 

[kf4eyr]

need to get away from the cab a short antenna will work better halfway down bed than against cab,,, a bumper mounted steel whip would work better,,, the best mount in my opinion is a tool box in the bed with antenna hard mounted a bout a foot to 18 inches away from cab with a steel whip or a top loaded or center loaded antenna,,,

 

[greusome1]

Just a side note... (Short time member, but comprehend very well. I read a lot of cb information online and I feel I'm getting very good at this hobby ++I like to test stuff on my own through trial and error.. yes I have smoked more than one radio).... So........Imho these guys are leading you in the right direction.... antenna too close to cab....(same as my atv sitting next to my house had very high swr with antenna literally inches from house where I parked it)... NEXT ISSUE THAT I SEE IS THIS WHICH WASN'T MENTIONED AT ALL:... anytime you have your coax cable anywhere above the antenna connection, you will have a greater swr reading...... looking at your pic.. i see that you have the coax running above the connector and back down in very close proximity... this is picking up some tx and putting it back into your radio (swr).... even a 102" whip suffers from this when placed on bumper as well heck... how you gonna stay below that right?... but it is usually not in very close proximity to the antenna... from your pic i would say if you ran the coax straight down instead of up and over.. your swr will lower.. prob not a lot.. but I bet you can tell a difference when using a swr meter..... just my 2cents worth......

 

[gewecke]

Homerjay, A Larsen Nmo27 in the center of your roof will out perform the box or mirror mounts. I know you said you didn't want it on your roof, but that's where your best performance will come from. 73, n9zas

 

[homerjay]

Hello. Sorry to revive an older post but I just w wanted to thank everyone for their help.

I have solved the issue, turned out I had a bad Antenna. The copper winding that was supposed to be soldered to the lug on the bottom was broken. Hence the off the chart swr I was getting. Once I changed Antenna to the other I get a reading of 1.5 on CH 1 and 1.2 on CH 40.

Can anyone explain the ratio to me? Eg 1.5:1

 

[prcguy]

VSWR is a complicated topic but if you use a simple restive mismatch as an example it might make some sense. VSWR stands for Voltage Standing Wave Ratio, and standing waves are the result of reflected power from an impedance mismatch between a transmitter and feedline and/or the feedline and antenna. Standing waves create higher and lower voltages at different points along the feedline compared to the voltage introduced into the feedline from the transmitter, with the peak variation being the result of the mismatch and the distance between points along the coax from high to low voltage as a function of frequency.

For example, if you have a 50 ohm transmitter and a 50 ohm feedline but place a 100 ohm resistor at the end of your feedline, it will introduce a 2:1 VSWR. This is the result of the reflected standing wave mixing in and out of phase with the original signal inside the feedline and producing a higher and lower voltage at different points along the feedline compared to the constant voltage inserted into the transmission line from a transmitter under a perfectly matched condition.

In this case of a 50 ohm transmitter and feedline with a 100 ohm load, the minimum and maximum voltage swing of the standing wave would be exactly two times what the original voltage from the transmitter would be under a perfect matched condition (2:1 VSWR). And as a coincidence 100 ohms is twice the impedance or resistance in this case, comparing a 50 ohm load to a 100 ohm restive load.

Unfortunately antennas and feedlines are not perfect resistors and there is inductive and capacitive reactance that complicate figuring things out on paper, but your VSWR meter is simple to operate and if used right will give you an idea of how your transmitter to feedline to antenna is doing.
prcguy

Hello. Sorry to revive an older post but I just w wanted to thank everyone for their help.

I have solved the issue, turned out I had a bad Antenna. The copper winding that was supposed to be soldered to the lug on the bottom was broken. Hence the off the chart swr I was getting. Once I changed Antenna to the other I get a reading of 1.5 on CH 1 and 1.2 on CH 40.

Can anyone explain the ratio to me? Eg 1.5:1