NGP SWR problems

JeffPro4x

Hot Pipe
Super Moderator
Supporting Member
Location
Glenside, PA
I had a 4' firestik mounted to my rear tire carrier and couldn't get my swr readings below 3.5. I bought that setup used, so I figured something might be wonky with it and trashed it. Now I'm running a 4' ngp firestik and have my swr at 2.2 on channel 1 and 40. Won't go any lower.

Here's what I'm running
Uniden pro505 cb
4' ngp firestik
K-4 stud mount
4" spring
17' ngp cable coupled to a 9' extension cable due to the ngp cable being too short.

The cable runs from the center storage box, under carpet, up pass side c pillar, across headliner, down and out behind the tail light and along the swing arm. With excess cable bow tied behind the rear cup holders.

Swr was tested in a wide open mall parking lot, as far away from the mall as I could get.

Any thoughts?
 

JeffPro4x

Hot Pipe
Super Moderator
Supporting Member
Location
Glenside, PA
Looks like I'm gonna have to do a mid channel reading too.

From http://www.firestik.com/Tech_Docs/swr-curves.htm

Narrow Bandwidth: After the antenna has been tuned and the SWR balanced on the lowest and highest frequencies [channels] and the SWR at the two reference points is above 2.5:1, a mid band SWR test should be performed. If it is below 1.5:1, then it is most likely that the antenna just doesn't have enough bandwidth to cover the entire frequency range of the radio. Usually, a physically longer antenna is needed. However, if the SWR is above 1.5:1 mid-band the most likely causes are insufficient ground plane supplied by the vehicle, insufficient or no chassis ground at the mount, too much of the antenna running parallel to parts of the vehicle or low quality and/or reactive issues with the coaxial cable that are often corrected by using 18-feet of good quality coax.

High & Flat: If the SWR is above 2.0:1 on the lowest and highest channels and is only a few tenths of a point lower at mid-band, the most common causes are insufficient ground plane, mounts that are not chassis grounded and/or really cruddy coaxial cable assemblies. There isn't much a person can do when it comes to insufficient ground plane. There is either enough, or not enough supplied by the vehicle. You can do a quick check of the chassis ground by simply clamping one end of a jumper cable to the antenna mount (not the antenna base) and the other to a known good chassis ground point on the vehicle then retesting the SWR. If it drops into a reasonable range and the mid-band SWR is now below 1.5:1, you know you have a chassis ground issue to deal with. A lot of mounts and/or mounting locations are either powder coated or painted, both which insulate the antenna stud mount and/or mount from chassis ground. If you are going to check chassis ground at the mount with a meter, make sure that the coax is disconnected from the radio to avoid a false reading via the radios circuitry. The mount and mounting surface must have chassis ground separate from that at the radio.
 

Fromfrontier2Xterra

I bought a Taco
Super Moderator
Supporting Member
Location
All over PA
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I really should learn what that all means. I hate paying someone to tune mine.

Sent via my walkie talkie
 

TJTJ

Skid Plates
Founding Member
Location
NJ
I had a 4' firestik mounted to my rear tire carrier and couldn't get my swr readings below 3.5. I bought that setup used, so I figured something might be wonky with it and trashed it. Now I'm running a 4' ngp firestik and have my swr at 2.2 on channel 1 and 40. Won't go any lower.

Here's what I'm running
Uniden pro505 cb
4' ngp firestik
K-4 stud mount
4" spring
17' ngp cable coupled to a 9' extension cable due to the ngp cable being too short.

The cable runs from the center storage box, under carpet, up pass side c pillar, across headliner, down and out behind the tail light and along the swing arm. With excess cable bow tied behind the rear cup holders.

Swr was tested in a wide open mall parking lot, as far away from the mall as I could get.

Any thoughts?








Its probably that your cabling is so long, and not a harmonic length.

17' is not 18' for example. 9' would be a harmonic length, but, you added it to a 17' cable.

You CAN improve the ground plane, as the GROUND plane is related to WHERE you mounted the antenna.

For example, the part of the ant ABOVE the roof line is all that's transmitting well forward. So, if you have a 4' stick on the carrier...a lot of that stick is below the roof line.

The MAXIMUM ground plane will be that stock dead smack in the middle of the roof for example...as the ROOF itself is the "plane".

On the carrier, the rear hatch is typically the plane, and its in the wrong orientation to be best....plus, the part of the ant over the roof...so the roof can be a plane too, but, with only a little height...and if that height is not a harmonic of the frequency, its not as good, etc....and to the rear, you have no plane at all.

Using a NGP cable doesn't make the SWR better per se, its just a band-aide. Adding a NON-NGP to a NGP cable, means more trouble, and so forth.
 
Last edited:

Mr Bills

Bought an X
Location
Area Code 530
Its probably that your cabling is so long, and not a harmonic length.

17' is not 18' for example. 9' would be a harmonic length, but, you added it to a 17' cable. . . .

Don't drink the CB cable length kool-aid. There is nothing magic about coax cabling in "harmonic lengths," i.e., quarter wave of 9', half wave of 18' etc. It is an urban myth repeated so often that it has become "fact."

The appropriate length of coax cable for a mobile CB is the shortest length necessary to get from the radio to the antenna mount without any sharp bends or kinks. It doesn't matter whether that measures out to 7', 9', 13', 18' or 23'.

These websites explain why better than I can:

http://www.stu-offroad.com/misc/myth-1.htm
http://www.radiomods.co.nz/coaxlength.html
http://www.cbantennaguide.com/Coax_Basics.htm
http://www.worldwidedx.com/threads/that-whole-coax-length-thing.57825/
http://www.bellscb.com/cb_radio_hobby/how_long_should_my_coax_cable_be.htm


 
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