Antenna tuning with Motorola R-2001D

[greg_kulosa]

I just got an R-2018D on eBay, and I am very happy with it. Can anyone with experience comment on the best way to tune mobile antennas with this machine? Say I wanted to tune either a CB antenna or a 2m/440 ham antenna mounted on a vehicle.

1 - Using the "Antenna Frequency" function of the "Cable Fault" and using the RF probe

or

2 - Using a return loss bridge (Say an Eagle) hooked up to the duplex gen set to 0mhz, and using the quasi-tracking generator with the spectrum analyzer.

Which one of these would give better results? I also have a Bird Wattmeter with the appropriate slugs, but that can be tedious if the antennas is starting WAY off of frequency, and I am not often sure which way I should be going.

Thanks very much.

[Wowbagger]

I cannot speak specifically to the R2100, but: you want to use the return loss bridge and the tracking generator. The resulting plot on the spec-an is basically 1/VSWR (not quite, but....) so the lower the notch, the closer to 1:1 the antenna. Think of it as making a bunch of readings over frequency with the Bird.

The only thing to look out for is that you need to get OUT OF THE NEAR FIELD of the antenna when doing the sweeps - when you are standing there you are going to alter the response significantly.

[greg_kulosa]

I cannot speak specifically to the R2100, but: you want to use the return loss bridge and the tracking generator. The resulting plot on the spec-an is basically 1/VSWR (not quite, but....) so the lower the notch, the closer to 1:1 the antenna. Think of it as making a bunch of readings over frequency with the Bird.

The only thing to look out for is that you need to get OUT OF THE NEAR FIELD of the antenna when doing the sweeps - when you are standing there you are going to alter the response significantly.

Thanks for the response.

BTW, the R2001D uses the sweep generator along with the RF probe to sweep around the center frequency. So it also gives a display of frequency versus RF energy. I just don't know which approach is more accurate, given that I have heard that the R2001D does not have a "real" tracking generator, just a "quasi" tracking generator (not really sure what that means).

I don't have a return loss bridge yet, and I'm trying to decide whether it's worth investing in one, or whether the RF probe will work just fine.

- N1NSY

[DJP126]

The duplex gen with zero offset will have too much internal feedback to get a reliable result. You'll be better off using the antenna tuner option with a GOOD RF probe and BNC Tee connector. I agree with Wowbagger that the bridge would be better but the R2000D series doesn't have a tracking generator.

[greg_kulosa]

The duplex gen with zero offset will have too much internal feedback to get a reliable result. You'll be better off using the antenna tuner option with a GOOD RF probe and BNC Tee connector. I agree with Wowbagger that the bridge would be better but the R2000D series doesn't have a tracking generator.

Ok, that's the information that I was looking for. So, no point in buying a return loss bridge at this point, then.

Is the Motorola RTL-4075A considered a "good" RF probe? That's what I have right now. Also, I am using an Amphenol brand BNC tee rated for 50 ohms, so hopefully that is a good BNC tee.

Thanks

-N1NSY

[greg_kulosa]

I forgot that I had another question - specifically for Dave:

The operators manual states that in the Sweep modes (either Sweep .01-1 or Sweep 1-10Mhz) that when the trace is in the middle of the display that the RF sweep is at the set frequency at that point. Is that calibration controlled with the same pot that controls where the spectrum analyzer center point is?

I ask because on my unit, the peak on the spectrum analyzer is not centered correctly. If I take the 10Mhz output from the back of the unit into the front, and set the unit for 10.000Mhz, the peak is off to the right of the center graticule. If I adjust that correctly, will that also guarantee that the Sweep frequency will be centered? Or is the Sweep center another adjustment point?

This is for an R-2018D/CP which I believe is the later "D" version.

Thank you for your help.

-N1NSY

[DJP126]

I forgot that I had another question - specifically for Dave:

Is the Motorola RTL-4075A considered a "good" RF probe?

The operators manual states that in the Sweep modes (either Sweep .01-1 or Sweep 1-10Mhz) that when the trace is in the middle of the display that the RF sweep is at the set frequency at that point. Is that calibration controlled with the same pot that controls where the spectrum analyzer center point is?

I ask because on my unit, the peak on the spectrum analyzer is not centered correctly. If I take the 10Mhz output from the back of the unit into the front, and set the unit for 10.000Mhz, the peak is off to the right of the center graticule. If I adjust that correctly, will that also guarantee that the Sweep frequency will be centered? Or is the Sweep center another adjustment point?

This is for an R-2018D/CP which I believe is the later "D" version.

Thank you for your help.

-N1NSY

The RTL4075A is OK but I never really considered it a good probe. In my opinion, it requires too much of a signal before it "fires" to get a DC voltage out of it. It will work for your application but you will have to use the high level RF (antenna) port with a level of -5DBm to +10DBm.

As for the centering of the spectrum analyzer, I would recommend having the extended alignment procedure done which calibrates everything in the unit. If you can get a copy of the R2001D MAINTENANCE manual, the procedure is in there. Don't worry that yours is a R2018D/CP, they are ALL the base R2001D. the 18/CP tells me that you have the cellular and trunking with coverage plus options.