Antenna analyzers, where and why?

[Tokin]

If you want a true representation of what the antenna is doing, you need to take your readings at the antenna feedpoint without any transmission line. Coax and its length does matter and will change the results. Or you can adjust the antenna with the transmission line connected and take the readings that include the influence of the coax, Its up to you. Im pretty sure the aa35 model has a setting that will eliminate the transmission line out of the equation when you take your readings. Check the manual and online. Bottom line is the true and correct way to tune an antenna is to adjust it to resonance at the operating frequency, note the resistance, connect your transmission line and compensate for any impedance mismatch between the antenna and coax with an impedance matching network.

Good shit right there. Thank you!
Its easy for me to take my antenna down and mount it on my 7' "tuning pole" I used to put the antenna together thanks to the pneumatic hinged tower in have. I can do whatever i need by myself. I left it there incase i needed to tune later or maybe build another antenna.

 

[sp5it]

Not always the case. An electrical 1/4 wavelength of coax acts as an impedance transformer. Connect a 1/4 wave stub to your perfect 50 ohm antenna and take a reading. Now you have a 2:1 swr.

2:1 really?

Mike

 

[Tokin]

Not always the case. An electrical 1/4 wavelength of coax acts as an impedance transformer. Connect a 1/4 wave stub to your perfect 50 ohm antenna and take a reading. Now you have a 2:1 swr. Huh? Impedance transformer, it can be used to your advantage if need be. Coax length DOES matter

Yeah, im seeing issues with the jumpers i made last week.

 

[groundwire]

The 2:1 is just an example, so he can get to thinking about these things, relax. If you want a true figure do the math.

 

[sp5it]

Since this analyser can't rule out coax from measurement, best way is to use 180° line to take measurements.
Mike

 

[Tokin]

I have no problem hooking directly to the feed point with a barrel connector. I was planning to put it on the pole this morning and see what i can do.

 

[Captain Kilowatt]

Not always the case. An electrical 1/4 wavelength of coax acts as an impedance transformer. Connect a 1/4 wave stub to your perfect 50 ohm antenna and take a reading. Now you have a 2:1 swr. Huh? Impedance transformer, it can be used to your advantage if need be. Coax length DOES matter

Ummmm.......no it does not. If the load is a pure 50 ohms no length of coax will change the impedance. A 1/4 wave of transmission line does invert the impedance such that in the case of a 25 ohm load which will present a 2:1 SWR, it will invert that so that the impedance becomes 100 ohms however the SWR still remains 2:1. ANY length of coax WILL act as an impedance transformer IF the load impedance is NOT 50 ohms. A very common application of this is when stacking two antennas that are 50 ohms each. Combining the two 50 ohm loads results in an impedance of 25 ohms. Using a 1/4 wave section of 37 ohm transmission line ( or two parallel pieces of 75 ohm coax) will result in what is know as a conjugate match and will transform that 25 ohms back up to 50 ohms. Coax line length DOES matter but ONLY when using it for impedance transformation or phasing. SWR is not affected by line length however the impedance presented to the transmitter does change along the length of coax. This is because there is an almost infinite number of R Xc and Xl combinations that will result in the same SWR.

I always use an electrical halfwave length of cable whenever possible to tune my antennas since that length repeats the impedance and virtually eliminates the cable from the equation.

 

[Tokin]

Do I need the ground plane kit on when tuning?

 

[groundwire]

Ummmm.......no it does not. If the load is a pure 50 ohms no length of coax will change the impedance. A 1/4 wave of transmission line does invert the impedance such that in the case of a 25 ohm load which will present a 2:1 SWR, it will invert that so that the impedance becomes 100 ohms however the SWR still remains 2:1. ANY length of coax WILL act as an impedance transformer IF the load impedance is NOT 50 ohms. A very common application of this is when stacking two antennas that are 50 ohms each. Combining the two 50 ohm loads results in an impedance of 25 ohms. Using a 1/4 wave section of 37 ohm transmission line ( or two parallel pieces of 75 ohm coax) will result in what is know as a conjugate match and will transform that 25 ohms back up to 50 ohms. Coax line length DOES matter but ONLY when using it for impedance transformation or phasing. SWR is not affected by line length however the impedance presented to the transmitter does change along the length of coax. This is because there is an almost infinite number of R Xc and Xl combinations that will result in the same SWR.

I always use an electrical halfwave length of cable whenever possible to tune my antennas since that length repeats the impedance and virtually eliminates the cable from the equation.

Oops

 

[sp5it]

Sure. You want to tune incomplete antenna?

 

[Shadetree Mechanic]

I would use the half wave coax to tune. If you connect with the barrel connector, you might influence the readings by being so close to the antenna.

 

[Tokin]

I would use the half wave coax to tune. If you connect with the barrel connector, you might influence the readings by being so close to the antenna.

Yeah well I don't have coax for that unfortunately. I'll have to use what I have about 80'.

 

[Tokin]

And I'm not happy at all with the screen omnthis rig expert. I will have to use it at night since you can't see shit during the day...

 

[Tokin]

And I'm not happy at all with the screen on this rig expert. I will have to use it at night since you can't see shit during the day...

 

[M0GVZ]

Not always the case. An electrical 1/4 wavelength of coax acts as an impedance transformer. Connect a 1/4 wave stub to your perfect 50 ohm antenna and take a reading. Now you have a 2:1 swr. Huh? Impedance transformer, it can be used to your advantage if need be. Coax length DOES matter

But you're not using it as a stub though unless it's connected inline by a T piece and then whether the SWR is changed or not is determined by whether the unconnected end of the stub is open or shorted.

If you're using a 1/4 wavelength of coax connecting the analyser to the antenna you should read 1:1 or as near as damnit.