Firefly antenna 3 ft vs 4 ft

[BobK]

In the real world on the Interstate and/or on the trail what will be difference in reception strength and distance with a three ft antenna on the back tailgate versus the four ft antenna?

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[Sting_NC_USA]

From what I've read, a difference in performance exists, but isn't substantial. Un-scientific comments seem to lead to about 1 to 2 miles additional range, possibly more if the antenna sticks up above your top (depends on where you mount, of course). I believe the 4-foot is more substantial in weight/heft. (Insert bigger is better quips here!).

 

[bryan]

This is an incredibly complex topic. The punchline is that where you place an antenna (especially the proximity of metallic masses to the base and radiating elements), the length and RF characteristics of the feedline, and bonding have as much to do with radiated power and patterns as the actual characteristics of the antenna itself. That's why you have to re-tune an antenna based on where you place it. An antenna that has a length that, unloaded, has a resonant frequency closer to the citizens' band will be more efficient. Without doing the math, I'm going to guess that's the 4' stick. But in a Jeep application, it isn't going to matter. The near impossibility of achieving a decent counterpoise means the efficiency difference of the antenna itself will be a very small part of the efficiency of the overall system.

Since the test rigs for this are expensive and complex (we're not talking a simple SWR meter here), we reduce things to simple principles that are easily applied: bigger is better; higher is better; clear space is better; metallic mass directly below is better; RF bonding of metallic masses to the RF ground is better (different from electrical grounding). But if your primary concern is how it looks or if you can pull into the garage for the 360 days a year you're not using the CB, don't worry about it.

 

[BobK]

This is an incredibly complex topic. The punchline is that where you place an antenna (especially the proximity of metallic masses to the base and radiating elements), the length and RF characteristics of the feedline, and bonding have as much to do with radiated power and patterns as the actual characteristics of the antenna itself. That's why you have to re-tune an antenna based on where you place it. An antenna that has a length that, unloaded, has a resonant frequency closer to the citizens' band will be more efficient. Without doing the math, I'm going to guess that's the 4' stick. But in a Jeep application, it isn't going to matter. The near impossibility of achieving a decent counterpoise means the efficiency difference of the antenna itself will be a very small part of the efficiency of the overall system.

Since the test rigs for this are expensive and complex (we're not talking a simple SWR meter here), we reduce things to simple principles that are easily applied: bigger is better; higher is better; clear space is better; metallic mass directly below is better; RF bonding of metallic masses to the RF ground is better (different from electrical grounding). But if your primary concern is how it looks or if you can pull into the garage for the 360 days a year you're not using the CB, don't worry about it.

I am thinking that my aftermarket steel rear bumper (JCR Offroad) may be a better location for the install, given that it is made of steel and is bolted to the frame with big steel bolts. Can't think of a single place with more steel other than the engine block. What do you think?

I already have several 1/2 inch holes to mount to on the bumper. The question then is rather than then try to get back into the vehicle with the coax back at the tailgate, do you think it would be OK to run the coax under the vehicle and up into the tub closer to the passenger footwell or even through the firewall?

 

[bryan]

I am thinking that my aftermarket steel rear bumper (JCR Offroad) may be a better location for the install, given that it is made of steel and is bolted to the frame with big steel bolts. Can't think of a single place with more steel other than the engine block. What do you think?

It doesn't have to be steel; aluminum will also make a counterpoise. And on the bumper, you'll have a whole bunch of metal "blocking" and "shaping" the RF energy (those terms aren't strictly correct, but communicate the idea). On the hood near the driver's A pillar is an OK choice with a Jeep, or go with a tailgate mount near the passenger side hinge.

Probably the best resource on the 'net for mobile radio installs is http://www.k0bg.com/. It's targeted at Ham radio and probably has more detail than you care to know, but it definitely applies to CB. If you want to dig deep, that's the place I recommend for more information.

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