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Author Topic: ATAS-120 with duplexer has High SWR  (Read 2805 times)
K6TKU
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« on: October 29, 2005, 08:53:15 PM »

I have recently purchased an FT-897 with an ATAS-120 antenna.  I added a Comet CF706 duplexer so that I could run all bands without coax switching.  I also installed the 6m/2m/70cm ground plane system from Yaesu.  The antenna is sitting on top of a tripod about 6 ft off the ground and fed with about 40 ft of RG8X.  Now here is my problem:

1. With the duplexer in the system the antenna presents a high (off the scale) SWR and the FT987 will not transmit.

2. When I take the duplexer out and run the antenna directly from the VHF/UHF connector on the back of the transceiver it seems to function properly.  The SWR is relatively low.

The duplexer was recommended by my supplier (HRO).  

Any ideas as to what I'm doing wrong?

Thanks...
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KG4RUL
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« Reply #1 on: October 29, 2005, 09:23:49 PM »

To get a clearer picture of your setup:  

I am assuming that you have attached the respective duplexer leads to the HF/50 MHz ANT� jack and the �144/430 MHz ANT� jack on the back of the rig.  

Then, I am assuming, you have a single length of coax going from the SO-239 jack on the rig to the SO-239 jack on a second duplexer.  

From there, the ATAS-120 is connected to the second duplexer HF lead and the VHF/UHF antenna to the other lead.

Am I correct in my assumptions?

Dennis KG4RUL
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KG4RUL
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« Reply #2 on: October 30, 2005, 04:48:11 PM »

Reset!

After I posted, I realized what your setup actually was.  You are using the ATAS-120 with the base kit for HF+ VHF/UHF!

What I believe is hapenning is that your duplexer is NOT passing the DC currents to the ATAS-120 from your transceiver.  Your rig uses DC in one polarity to dirve the antenna motor in the up direction or reverses it to drive in in the down direction.  If the Duplexer does not have specific circuitry to pass-through the DC current, the antenna never moves and your SWR stays high.

The sequence is something like:

You press the tune button and rig remembers your current power level and operating mode.  It then sets the mode to CW and sets the power to a low value approx. 10 watts.  It then starts the antenna moving in the appropriate direction, based on the last frequency you auto-tuned to, until the minimum SWR is achieved.  The motor stops and the rig returns to the previous mode and power level.

Because the rig has some logic built-in to it's prgramming, the antenna does not need a turn counter for the tuning motor, as other antenna schemes do.  Unfortunately, to use an ATAS with other than Yaesu rigs, you need a controller with some smarts built into it such as the turbo-tuner interface.

Dennis KG4RUL
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K6TKU
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« Reply #3 on: October 30, 2005, 06:24:35 PM »

Hi Dennis.. Thanks for getting back to me so fast.  It turns out that the duplexer does pass the DC signal because the antenna goes through its up/down cycle trying to reach min swr.  When I connect the antenna directly to either xcvr output the thing cycles and the swr goes through minimum but keeps right on going.  I can manually tune it up/down from the xcvr.
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KG4RUL
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« Reply #4 on: November 01, 2005, 04:58:44 AM »

Sounds like you may have a problem in the radio circuitry.  Have you used an external SWR meter to see what is actually going on as the antenna tunes?

Dennis KG4RUL
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KK4BMI
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« Reply #5 on: July 16, 2013, 06:32:25 PM »

Dave KK4BMI
If your using it as a base antenna I had the same problem. Try investing into this for $99.95 Yaesu ATBK-100 VHF/UHF Antenna Base Kit for ATAS120
Not required unless operating the VHF or UHF bands with the ATAS-120 off the vehicle. It restores the low-angle response for enhanced performance. Use with suggested base mount. You can purchase it through AES Electronics. Thanks
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W9IQ
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« Reply #6 on: July 17, 2013, 04:49:21 AM »

Jim,

You didn't say but I assume you have the low pass side of the diplexer connected to the HF antenna connector on the FT-897 and the high pass side connected to the VHF/UHF connector. It might be helpful to know what band you were trying when you experienced the high SWR. Did the antenna "retune" when you connected it directly to the VHF/UHF connector? Was it fully retracted in both cases?

You may wish to try disabling the ATAS tuning and substitute a dummy load on the output of the diplexer. This will tell you if the diplexer is defective.

Did you already setup the tuning menu (N0-085 [TUNER/ATAS]) on the FT-897 to be "ATAS(ALL)"?


- Glenn W9IQ
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