antenna light coming on cobra 29

A 'certified' technician will not agree 4 feet will properly 'co-phase' at 27 MHZ unless they are 'certifiable'. Opinion cannot alter the math and physics involved and 4 feet is only a quarter wave or odd multiple thereof in specific instances of frequency and velocity factor which are never going to happen in the 11 meter band. To phase properly two 50 ohm antennas you must use an odd multiple of quarter waves of 75 ohm coax (72) given proper frequency and velocity factor, with the antennas a quarter wave apart. You can insert a quarter wave of 50 (52) ohm coax in one leg as a phase delay line if you wish to rotate the pattern, direction selected by picking which leg you insert it in VS the orientation of the two antennas. This idea is used for creating a broadside array and has been used to achieve circular polarization. If you do measure a low SWR with 4 feet this merely speaks to reactance, feedpoint impedance, and counterpoise considerations but I assure you the field pattern does not contain the wave fronts showing constructive and destructive interference patterns needed to be properly 'phased'. A low SWR does not provide information concerning the pattern in the far field radiated by the system. Nor does it indicate what portion of the overall feedpoint impedance is actual radiation resistance. In simpler terms this is why a dummy load may look good on the SWR meter but it is not going to talk very far down the road.


The reason designers tune antennas and spec them out with 18 feet of 50 ohm coax in mind is simply due to the fact that these whips are not 50 ohms. They are closer to 32-36 ohms. Radials are drooped 45 degrees to raise this feedpoint impedance but this is not workable in most mobile applications. If the load were a perfect resistance with zero reactance then the only consideration coax length would have is loss. Problem is there is always some reactance so lengths other than tuned halfwaves become transformers, used often in conjugate matching. Lack of understanding this factor is where all the disputes come from concerning coax length. Short as possible is better assuming other factors such as reactance and quality of counterpoising do not alter the equation concerning actual SWR in a real world application. However this cannot alter the need for an odd multiple of quarter waves of 75 ohm coax when phasing is being considered.

 

A 'certified' technician will not agree 4 feet will properly 'co-phase' at 27 MHZ unless they are 'certifiable'

I need a set of keys to that padded room ....

 

Merely say 4 feet is a quarter wave at 27.205 MHZ with common coax and they will let you in for free. 246/27.205 = 9.042 feet, times VF = actual length in feet. To get 4 feet you would need coax with a velocity factor of 44.2357 percent. Which Beldon number do you claim that is?

 

Wew ..... you had me worried with that 4 foot stuff you were talking COAX not free space

 

I was actually Reading the thread comprehensively and responding to a comment made by another previously. Not partially focusing in on a few words from a line paying no attention to the overall subject matter. To do otherwise is to create error and misdirection.

 

I know but it has been a bad day in the radio repair ( SMT board level bad parts ) world here and I needed a chuckle

Back to the bad radios

 

You only need a chuckle consider yourself lucky. I need an electron microscope I can't see that tiny crap any longer.

 

NO KIDDING send me one too

 

Check your SWR with another meter. It could just be your AWI needs adjusting. VR-6 if it's a Cobra 29LTD.

 

TECHNICAL SERVICE BULLETIN
10057
MODEL(S):
CB model 29 (all versions with the ANT light)
DESCRIPTION:
The ANT light is coming on even though the SWR is b
elow 3.
The specification for the ANT light is as follows
:
•
The ANT light should not illuminate if the SWR is b
elow 3.
•
The ANT light should illuminate by the time the SWR
reaches 6.
ACTION:
Trimmer potentiometer VR6 will adjust the antenna l
ight. VR6 is located in the
back of the radio, between the antenna jack and the
power jack.
Equipment required:
25 ohm non-inductive load capable of dissipating 4
watts
1KHz audio source
Modulation meter
ADJUSTMENTS:
Adjust as follows:
1. Connect the 25 ohm load to the antenna jack.
2. Turn VR6 fully counter-clockwise.
3. Key the microphone and apply enough audio for ##
ll modulation.
4. Slowly turn VR6 clockwise just until the antenna
light turns off