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FT-818 Odd SWR Readings on 70 cm

 

My FT-818 shows 3 bars on the SWR meter on 70 cm at the 5 W setting, when connected to a good quality UHF/microwave dummy load.? On 50 or 144 MHz it shows zero bars.? This is true for both front and rear antenna connectors.? The output power is indeed 5 watts, measured on a Coaxial Dynamics meter.? I have tried a couple of good dummy loads.

Can anyone comment as to whether this is normal behaviour?? A local says his FT-817 shows zero bars on? 432 MHz on a good load......so is there a change from the FT-817, or is there something amiss in my '818?

73,
Steve VE3SMA
 

Just a thought Steve? - are you using a patch lead to the dummy load - can you swap for another?

I've an 817 and really thinking about it never used 70cm except to listen on.

Dave G0CER


On Thu, 25 Apr 2019 at 16:00, sjkavanagh1@... [FT817] <FT817@...> wrote:


My FT-818 shows 3 bars on the SWR meter on 70 cm at the 5 W setting, when connected to a good quality UHF/microwave dummy load.? On 50 or 144 MHz it shows zero bars.? This is true for both front and rear antenna connectors.? The output power is indeed 5 watts, measured on a Coaxial Dynamics meter.? I have tried a couple of good dummy loads.

Can anyone comment as to whether this is normal behaviour?? A local says his FT-817 shows zero bars on? 432 MHz on a good load......so is there a change from the FT-817, or is there something amiss in my '818?

73,
Steve VE3SMA





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73 Dave H G0CER
 

I used completely different cables for the front and rear antenna connectors, so I think I have taken the bad cable possibility out of the possible reasons list.? I also operated for a while on 70 cm yesterday and consistently got the 3 bars SWR reading on a well-matched antenna (with another completely different piece of coax).? It seemed to be working fine, apart from the odd SWR reading.

73,
Steve VE3SMA
 

Go for an external meter then.? I'm using my 817 this weekend if I have a 70cm antenna I'll check it but mostly I just use it for HF.

Dave G0CER


On Thu, 25 Apr 2019 at 17:34, sjkavanagh1@... [FT817] <FT817@...> wrote:


I used completely different cables for the front and rear antenna connectors, so I think I have taken the bad cable possibility out of the possible reasons list.? I also operated for a while on 70 cm yesterday and consistently got the 3 bars SWR reading on a well-matched antenna (with another completely different piece of coax).? It seemed to be working fine, apart from the odd SWR reading.

73,
Steve VE3SMA




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73 Dave H G0CER
[email protected]
 

Make sure that you have selected the same antenna connection for UHF as VHF or move the dummy load to the other antenna port. You can independently select one to the front BNC and the other to the back SO239.
 

Steve, the problem may be parasitic capacitance if the dummy load has multiple resistors due to the resistor lead end caps, the more resistors the worse the problem - check out QST article on UHF dummy load, it shows 11 x 560 ohm resistors in parallel and a small coil - looks like a very good article.
Regards
73 Paul 2E0YOB

Paul Simmonds
 

When I say a good dummy load...I mean a Bird 8080, rated to 3.5 GHz.? Same symptom on another good dummy load and on an antenna known to be well matched.

73,
Steve VE3SMA
 

There are settings in the service menu (the "hidden" menu) to adjust the reflected power sensitivity for HF, VHF, and UHF.? Perhaps those got misadjusted or corrupted (or perhaps the rig came from the factory misadjusted).? I don't remember which menu setting and I am at work so can't check my notes, but you can Google it.

Eric Forman
KM4VR
"Vinum lux Solis est, ab aqua coercita" - Galileo


On Fri, Apr 26, 2019 at 8:17 AM sjkavanagh1@... [FT817] <FT817@...> wrote:
?

When I say a good dummy load...I mean a Bird 8080, rated to 3.5 GHz.? Same symptom on another good dummy load and on an antenna known to be well matched.


73,
Steve VE3SMA
 

Thanks, Eric.? Assuming the menu is the same in the FT-818 as in the FT-817, it appears that the parameters available there are for the sensitivity of the power foldback (high SWR protection) to the SWR reading, rather than the calibration of the SWR meter itself, but the details I found are a bit sparse.

KA7OEI did point out that on his FT-817 (original) a perfect load gave 2 bars of SWR reading at 5 W on a perfect load.? He found the adjustment instructions for this in the service manual, but it didn't actually work without circuit modifications.? It may be that my FT-818 is just a bit worse!

73,
Steve VE3SMA
 

Re: SWR. My FT817ND was purchased new in early 2006. The SWR readings don't match this table:

On mine there are no SWR bars on 145 MHz, but 2 to 3 bars on 435 MHz with a dummy load good to over 1GHz. The rear and front connectors don't make much difference.

Note SWR due to higher apparent resistance rather than lower is less of a risk, the 2:1 could be 100 Ohms or 25 Ohms, but 25 Ohms is higher current. Most SWR meters don't tell you which it is. Also an open circuit on the end of a transmission line will look like a short circuit at specific lengths and frequencies. I have a 50 Ohm load that can be connected as "through load" and with a 50 Ohm on that and an electrically short cable you then have 100 Ohms = 2:1 (which is safe) to check your SWR meter(s).

The service menu settings don't calibrate the SWR metering.

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