High SWR reading (5)
04-02-2014, 07:00 PM
#21
JK Super Freak
The antenna "length" is really about the length of wire wrapped around the fiberglass rod. The fiberglass rod is just a support structure for the wire.
A 3 and 4' firestick, for example, have the same length of wire wrapped around the outside. As the fiberglass rod gets shorter, it's wrapped tighter and loses efficiency.
The most efficient would be to not have it wrapped at all... a full size 109" whip for example, which is roughly a 1/4 wave at CB frequencies. But you're fine. It's a CB. It doesn't need pure efficiency for typical CB purposes.
04-03-2014, 06:02 PM
#23
JK Newbie
Join Date: May 2011
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This is almost always a "grounding" issue, which is to say, the mount and connection of the mount to the vehicle is not allowing it to use the body as the counterpoint or ground for the vertical monopole, what you'd call the antenna.
If the coax is good and the different lengths cause different SWR readings, that means the system is using the coax as the counterpoise rather than the body/frame of the vehicle.
Try to scrape off some paint on the back of the mount (side against the body of the vehicle) and scrape some paint off the body that faces the part of the mount that you scraped off. This should allow a clean electrical connection and should also be a better RF connection.
You might try a little dielectric grease where you scraped it too.
If the coax is good and the different lengths cause different SWR readings, that means the system is using the coax as the counterpoise rather than the body/frame of the vehicle.
Try to scrape off some paint on the back of the mount (side against the body of the vehicle) and scrape some paint off the body that faces the part of the mount that you scraped off. This should allow a clean electrical connection and should also be a better RF connection.
You might try a little dielectric grease where you scraped it too.
arjeeper is correct in referring to the Jeep body/frame functioning as a counterpoise. The term counterpoise goes back to the beginnings of radio and is a construct, usually of wire, either above ground, or buried below ground, that is connected to the transmission line ground. It functions as an aid to couple the antenna system to earth ground. Think of it as a large coupling capacitor. This is always used with a monopole and helps establish the ground plane. Think of an inland AM commercial radio station tower.
The Jeep frame / body also couples the monopole ground to earth ground (and the only functional ground plane) via capacitive coupling. The average 2 door jeep with a 4 1/2 inch lift forms a .001 ufd capacitor coupled to ground. This value of capacitance has an impedance of about 5 ohms at 27 Mhz, so this is a very lossey ground path. What should be a theoretically simple antenna system becomes more confused because the monopole is fed as a balanced dipole, by an unbalanced feed line (coaxial cable). Any additional antenna theory detail is counterproductive and not appropriate to the average CB'er.
I have to disagree with the statement that the coaxial cable acts as a counterpoise, since the definition of counterpoise precludes that, unless a "no ground plane" system is intended. This is not within our definition of "simple transmission line. The power that flows down the outside of the cable from the antenna is called "common mode current" and is a normal byproduct of an unbalanced transmission line feeding a radiating load that reflects power back to the source. I also disagree with the concerns expressed regarding coiling excess coax as being detrimental to the operation of the system. The signals inside the coax are immune to external influence within the definition of a simple transmission line. (if some one can supply a reputable source stating otherwise (no antenna manufacturer promo lit) I would like to see it) There is no downside as common mode current is wasted power returned from the antenna. In fact it is common procedure to coil the coax outside the vehicle, in conjunction with appropriate ferrite core material to form a choke to block the entrance of common mode into the cab. This technique seems to work and therefore contradicts an alternate contention that common mode is RF energy that was produced by the source and never delivered to the antenna. This could be a conflict of semantics and regardless, is a distinction without a difference.
In addition some clarification is needed regarding coax length vs measured VSWR. In a simple transmission line as we commonly use in off roading, the length of the coax has no bearing on the system VSWR. Now to confuse the issue, necessarily, the observed VSWR can be in error when measured at the power source, the back of the radio. The readings can appear better than they really are because of loss characteristics of the cable. The only place VSWR can be measured with out this error potential is at the base of the antenna.
Other than loss, cable length has no affect what ever on the operation of a simple transmission line. Coaxial cable physical length and its electrical length are significantly different because of a property called velocity factor. RF travels through a transmission line slower than thru free space. This is the Vf. The cable we use most has a Vf of .66 /.67. This means that the wavelength of the signal is %34 shorter while traveling inside the coax. Thus, the true half wavelength of a coax, at 27 Mhz is about 10.94 feet, depending on the Vf, NOT 18 feet as is the common misconception.
Citations, below, for those who care about accurate info.
http://www.stu-offroad.com/misc/myth-1.htm
http://www.1728.org/freqwave.htm
http://n1gnn.com/CoaxVelocityFactorLength.html
http://www.hamuniverse.com/coaxdata.html
http://en.wikipedia.org/wiki/Velocity_factor
http://www.arrl.org/files/file/Techn...f/q1106037.pdf
http://www.radio-electronics.com/inf...ity_factor.php
http://www.stu-offroad.com/siteindex.asp
Phil
Last edited by psouza; 04-04-2014 at 01:21 PM.