Power/VSWR Indication on the FT-817:
Like that of many transceivers, the power and VSWR meter readings on the FT-817 are relative. That's to say, a higher reading means higher power - and that is all. It doesn't necessarily mean a certain (calibrated) wattage, VSWR. It is fair to say that these readings, while not meaning anything specific, are likely to be repeatable given identical operating conditions on the same radio. (In other words, given a 1:1 VSWR, you could conceivably calibrate the power meter to correlate with a specific output power.)
There have been several attempts made to quantify the VSWR meter readings with actual VSWR conditions. Prior to going into this, it must be completely understood that VSWR can result from several types of mismatch:
Any type of VSWR determination circuit will respond in its own way to a combination of one or more of the above conditions. While it is certainly possible to construct a circuit that will take into account any of the above conditions its relative complexity dictates that such a circuit is used only when such measurements are specifically required. Much simpler circuits will certainly give results that will allow the intended design goals (i.e. protection of the final amplifier stages.) One common trait is that any of these circuits should be able to read forward and reverse power more correctly as the VSWR approaches 1:1.
The FT-817 employs three circuits to determine forward and reflected power: One for HF through 6 meters, one for VHF (2 meters) and one for UHF (70 cm.) These construction of these circuits are quite typical: The HF/6 meter circuit is the typical "one turn primary" toroidal detector (used in most HF wattmeters) while the VHF and UHF circuits are circuit board "stripline" types - typically used for narrowband power metering.
It should also be understood that the '817 doesn't really read SWR. To do that, it would have to measure the forward power, the reflected power, and then do a math calculation. What it does indicate is the amount of reflected power. For example, were the SWR 3:1, you'd expected to see 25% of your power coming back at you, or 1.25 watts with 5 watts forward power. If you were to set the '817 to output 1 watt, then you could have an infinite SWR yet still the meter wouldn't necessarily read any higher than it did with the 3:1 mismatch at 5 watts. Why? With only 1 watt output, it'll never detect 1.25 watts coming back! (Note: In real life, with extremely bad mismatches, the detecting circuit will probably not be accurate and could read much higher or lower forward/reverse power than actually exist. In other words, they can't be trusted to be accurate under severe mismatch conditions!)
I decided to make some measurements using purely resistive loads placed directly at the front BNC antenna connector. These readings were made at a supply voltage of 12.5 volts using high power.
When making these measurements, it immediately became obvious that something was amiss: As you can see, the VSWR indicator showed that with a perfectly good 50 ohm load (I used a known good 50 ohms resistive load rated to 4 GHz) on band 6 meters and up, a "flat" (1:1) VSWR was not indicated. In fact, on 6 meters, the "best match" seemed to be (according to the '817's meter) at about 27 ohms or so - an actual VSWR of about 1.8:1. On 2 meters and 70 cm, there seemed to be no resistance that provided what was, according to the meter, a "perfect" match.
This sort of result indicates a misadjustment in the VSWR bridge in the radio, a problem with the design, or a combination of both.
Being curious, I opened the radio and the service manual and followed the adjustment procedure. For HF/6 meters, the procedure called for adjusting trimmer TC3003 on 10 meters for lowest voltage at test point TP3001. I also experimented with doing this procedure on 6 meters, instead. I noticed that doing so on 6 meters resulted in perfect operation on that band, but a false high VSWR indication on other HF bands. Doing the adjustment on 10 meters again resulted in a different result on 6 meters. A little bit of experimentation resulted in being able to find a setting that seemed to work properly on all frequencies HF through 6 meters.
On 2 meters, the story was slightly different: I couldn't get the voltage at TP3001 below about 0.23 volts. I then tried 70cm, but noticed that the best null at TP3001 occurred when the capacitor was at minimum. Hmmm... I removed C3284 and noticed better results... What I needed was negative capacitance - which could be effectively simulated (in this case) with inductance - so I replaced C3284 with a small inductor (about 4 turns of 30 wire on a 1/8 diameter form) and with a bit of adjustment of the capacitor and inductor (alternatively) I was able to get the circuit to work perfectly. Owing to the circuit board layout and the lack of a similar capacitor in the VHF circuit, I chose not to try this method on that circuit, but I'm sure that it would work.
Anyway, because I now believe that re-making such a chart
be relatively worthless (other than for my radio) I decided not to
the chart after the adjustments and modifications.
Just because the radio has "High VSWR" protection, don't assume that this makes the radio indestructible!
Actually, most SWR protection circuits (but not all) do not detect SWR per se - they simply detect reflected power.
Suppose that your SWR protection kicked in at about 25% reflected power at 100 watts (about 3:1 VSWR.) If you were to drop your power to 50 watts, you'd need 50% of your power to come back at you (or a VSWR in the area of 5:1 or so) to trigger the SWR protect circuit - and at less than 25 watts, you would never be able to trigger the SWR "foldback" circuit at all.
It isn't the SWR that can damage the radio - but the possible combinations of excess heat and higher voltage (on the final amplifier's devices) that can cause the damage.
This brings up another point: By the time the SWR protect circuit detects high reflected power, there has been - for a brief instant- much higher power - It takes a finite amount of time for the circuit to detect that excess reflected power and shut down the radio.
What this means is that if you insist on operating your radio at full power with a bad load, you are intermittently subjecting its finals to high VSWR conditions - which can include voltage in excess of the final's ratings.
If you keep doing this, you can damage or even destroy your finals.
In the case of the '817 - where it is possible to get more than the rated 5 watts out of the PA - the best advice is: don't do it! You are just asking for trouble if you do this modification!
"What will a bad SWR do to my radio, anyway?"
Probably, not much. Despite its flaws, this circuit certainly offers protection against gross mismatches, but it does not provide a useful means of determining the actual match conditions: Use a real VSWR bridge for this.
If you were to operate the FT-817 with a poor VSWR, the result would likely be low radiated power: Firstly, the '817 will limit its maximum power commensurate to the VSWR conditions. Secondly, a high VSWR usually means that any coax in the line will be far more lossy than it would be under matched conditions. In other words, you won't get "out" very well.
Will the SWR damage the radio? Probably not directly: The MOSFETs output transistors used in this radio are being operated very conservatively and will probably not be damaged by a mismatch. What is more likely to cause damage is overheating: Normally, the 5 watts output by the radio will go into the antenna, and out into the universe. If the VSWR were bad and the radio didn't reduce its power for some reason some portion of that 5 watt output would be "eaten" by the FT-817, resulting in that much more heat to get rid of.
In general, if the FT-817 indicates that it is happy with the match
(i.e. low SWR) then you are not likely to damage the radio.
Go to The KA7OEI FT-817 "Front Page" - This is, well, the "front" page of the '817 pages here...
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This page maintained by Clint Turner, KA7OEI and was last updated on 20041110. (Copyright 2001-2004 by Clint Turner)