Calling all radio gurus

[Walker]

I just bought a field-strength/SWR meter from radio shack today to test my new Uniden CB. I have a K40 mounted on top of my headache rack, and the cable running under the truck to the engine bay, through the firewall into the cab.



I tested the SWR on ch. 20, 1, 40, and 19. The highest I got was a 1. 3. So, that checked out great. But, when I went to check the field strength, I could barely pick up any signal (scale is 1-5 on the meter, I could barely get a 1. 5 about 10ft. away) standing anywhere within the 25ft. circle required.



My truck was parked away from the garage, facing east, on a slight incline, basically out in the wide open. I've never tried to test this before, so??? When I stood in the bed of the truck, level with the ant. about 8ft. away, I got a 4 once. Got down, walked around the truck again and never got above a 2 anywhere close to the truck (in the 25ft. circle again).



I thought my ground could be bad on the headache rack, since it is sprayed with bedliner, so I moved the mag. mount ant. to the roof of the truck for a test. Still nothin' (no improvement).



How can my SWR be so danged good, and my field strength be so weak? I thought SWR tested how much % of signal was getting out. So, with SWR so low, why can't I pick up more signal on the field strength meter?

 

[fest3er]

Originally posted by Walker

... How can my SWR be so danged good, and my field strength be so weak? I thought SWR tested how much % of signal was getting out. So, with SWR so low, why can't I pick up more signal on the field strength meter?

The standing wave ratio (SWR) is a measure of how well the antenna is electrically matched to the transmitter. For instance, if the transmitter wants to be matched at 50 ohms impedance, and the antenna is, electrically, 50 ohms impedance at the desired frequency, the SWR would be 1, meaning that all of the energy from the transmitter is sent to the antenna.



As the antenna's electrical parameters change, the SWR will increase, meaning that some of the transmitted signal (power) is reflected back to the transmitter, then back to the antenna, then back to the transmitter - hence the 'standing wave' monicker. Any impedance mismatch will cause some of the signal to be reflected away from its destination. (As a side note, this is true of just about *any* electrical signal transmission - E1, T1, telephone, RF, ethernet, PCI bus, ISA bus, et al; an impedance mismatch always causes some of the signal to be reflected. )



The antenna's impedance changes with frequency and physical length. Thus you might have an SWR of 1. 3 at channel 19, but an SWR of 4 at channels 40 and channel 1.



One reason you are not seeing much field when the SWR is 1. 3 is that the transmitter is not putting out as much power as you might think. Another reason could be that the antenna just doesn't radiate efficiently.



Good luck,

Fest3er

 

[Rob Thomas]

Yea,... ... ... ... ... ... ... ... ... ... . I was gonna' say that. :-laf

 

[Sregorb]

Also...

Keep in mind also that your field strength is going to be well ABOVE your ground plane. So if your antenna base is at 4 feet your field strength is generally going to start at 4 feet high and work upwards from there. Not to get into the murky theory but yes, there is a little bleed lower than take-off angle but it tends to be reflected back up (vertical polarization) or absorbed (horizontal polarization) by the ground if it hits. That's why you'll want the ground plane (where the base sits) to be as high and unobstructed as possible--to get as much signal high and into the air.

Climbing down from the bed (closer and higher) can significantly drop any received signal, thus your lower readings. Try same test with receiver higher (like on ladder or side of hill nearby).

Hope this helps. Lemme know.



Brian

 

[CAM]

:--)

 

[Walker]

Thanks for the replies. My ant. is mounted at the highest pt. on the vehicle, the headache rack, which puts it close to 6. 5-7ft in the air. So, maybe walking around at ground level, I wasn't high enough to pick up the transmission signal. But, the instructions showed how to make a grid and measure the signal direction based on walking around (ground level) and marking the strength readings. However, this example did say it was based on a "trunk mounted" ant. on a car.



Hopefully the ht. is the only issue for reading the signal and everything works fine. I'll test it on the open road tomorrow (600 mile trip).



Thanks again.

 

[PRout]

Yea, most of your power is going to be radiating upwards. Also, alot of fiberglass whips are wound tighter at the top so alot of the energy is radiating from up higher.



We used to fire up the amps and walk around the truck w/ a neon light, and the light lights from the radiated energy. Can go up to 30 ft away holding a flourescent tube in your hand & if you can hold it high enough, it glows incredibly bright. Blows peoples minds to see that. It helps when you have 3-4kw output though. .

 

[Sregorb]

hehe, Cam- was that for me???

:--)



Brian

 

[The patriot]

I'd run a 10 gauge wire from bare metal on the rack, to bare metal on your frame.

Dont trust the rack to ground itself on the bed just because it's in the bed posts.

Eric

 

[Rotty]

The CB whip probably is launching the RF signal at >45 degrees up. Also, check the a RF power meter, like a bird watt meter to check the actual rf output power in watts. A 1/4 wave antenna has the main lobe of rf power at approximately 45 degrees ( the old metal CB whips of 102" are 1/4 wave). A 5/8 wave antenna has the main lobe at approximately 5-10 degrees from horzintal. So probably what is happening is you are in a null of the RF field and will see very little field strength. Good luck... . KD4KIY

 

[BK]

just another bit of info.

The field strength measurement on these SWR meters are just a reference measurement. Also on the two oldies inexpensive units I have, the calibration knob for adjustment of the SWR measurement also adjusts the sensativity of the field strength circuit.



Most radios today don't have much of an issue about producing close to legal power transmittion limits.

Most also use the S meter for transmittion power measurements. And with your SWR being fine, if your at or near max (usually a red bar or segment or the old Cobras had 1-5 watts), your power out is probably ok to.