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P3 TX Mon Power Measurement Anomaly

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Ken Widelitz

P3 TX Mon Power Measurement Anomaly

I am having an anomaly with the TX Mon power measurements on my P3s. The
power varies significantly depending upon where I put the TX sensor in the
RF chain (K3 -> BPF -> Alpha 87A -> Coax Stubs -> Tuner -> Antenna Coax.) I
have a 3" coax jumper on the appropriate end of the P3's TX sensor. I get
similar results on both of my radios connecting the TX sensor in the same
place in the RF chain. For instance, on 20 meters, with the K3 set to full
power and the Alpha 87A in standby, the P3TX Mon displayed power is as
follows:

TX Mon Location Watts

@K3 RF Out 106 -> 6' coax ->

@ BPF Input 101 That is a 5 watt measured
decrease in a 6' piece of RG-213.

@BPF Output 92 That is a 9 watt measured decrease
through an Array Solutions W3NQN design BPF. -> 6' coax ->

@Alpha 87A RF Out 86 That is a 6 watt measured decrease in a
6' piece of RG-213 and through an Alpha 87A in standby.

The Alpha 87A RF Output LEDs show just over 100 watts (the 4th LED
flickers.) That indicates to me the TX Mon power displayed is incorrect.
When the Alpha 87A shows full output (all green LEDs lit) the TX Mon reads
1329 watts. That is a 13+% difference. I don't have any other power meter as
a reference.

I have measured both coax jumpers and the BPF individually using an AIM 4170
and a 50 ohm load. The SWR is flat at virtually 1:1 for each. The SWR
displayed on the P3 is almost the same when measured at the K3 (1.53:1) and
on the AIM 4170 at the end of the coax to the antenna (1.48:1.)

Does anyone have a good explanation for this? I know there is a calibration
method for the TX Mon, but I that doesn't explain the measured power drop in
the RF chain with the amp in standby.

73, Ken, K6LA / VY2TT

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k6dgw

Re: P3 TX Mon Power Measurement Anomaly

"> Does anyone have a good explanation for this?"

I don't know if this is a "good" explanation, but it is one that is
sometimes [often?] true:

Many TX Power sensors actually measure voltage, it's a whole lot easier
than power. The meter or display is marked in watts which are computed
by the designer from E^2/Z. Z is assumed to be constant, usually 50+j0
ohms. That's rarely the case in reality.

Moving the sensor up and down the equipment-coax chain will subject it
to slightly different values of Z, ergo E^2/Z changes.

73,

Fred K6DGW
- Northern California Contest Club
- CU in the Cal QSO Party 1-2 Oct 2016
- www.cqp.org

On 12/28/2015 4:50 PM, Ken Widelitz wrote:

> I am having an anomaly with the TX Mon power measurements on my P3s. The
> power varies significantly depending upon where I put the TX sensor in the
> RF chain (K3 -> BPF -> Alpha 87A -> Coax Stubs -> Tuner -> Antenna Coax.) I
> have a 3" coax jumper on the appropriate end of the P3's TX sensor. I get
> similar results on both of my radios connecting the TX sensor in the same
> place in the RF chain. For instance, on 20 meters, with the K3 set to full
> power and the Alpha 87A in standby, the P3TX Mon displayed power is as
> follows:
>
>
>
> TX Mon Location Watts
>
> @K3 RF Out 106 -> 6' coax ->
>
> @ BPF Input 101 That is a 5 watt measured
> decrease in a 6' piece of RG-213.
>
> @BPF Output 92 That is a 9 watt measured decrease
> through an Array Solutions W3NQN design BPF. -> 6' coax ->
>
> @Alpha 87A RF Out 86 That is a 6 watt measured decrease in a
> 6' piece of RG-213 and through an Alpha 87A in standby.
>
>
>
> The Alpha 87A RF Output LEDs show just over 100 watts (the 4th LED
> flickers.) That indicates to me the TX Mon power displayed is incorrect.
> When the Alpha 87A shows full output (all green LEDs lit) the TX Mon reads
> 1329 watts. That is a 13+% difference. I don't have any other power meter as
> a reference.
>
>
>
> I have measured both coax jumpers and the BPF individually using an AIM 4170
> and a 50 ohm load. The SWR is flat at virtually 1:1 for each. The SWR
> displayed on the P3 is almost the same when measured at the K3 (1.53:1) and
> on the AIM 4170 at the end of the coax to the antenna (1.48:1.)
>
>
>
> Does anyone have a good explanation for this? I know there is a calibration
> method for the TX Mon, but I that doesn't explain the measured power drop in
> the RF chain with the amp in standby.
>
>
>
> 73, Ken, K6LA / VY2TT

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Ken Widelitz

Re: P3 TX Mon Power Measurement Anomaly

In reply to this post by Ken Widelitz

Hi Mark,

Thank you for a cogent explanation. Clearly, I never thought about the math
nor realized how much those little losses add up.

73, Ken, K6LA / VY2TT

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Mark Goldberg

Re: P3 TX Mon Power Measurement Anomaly

I certainly understand the concern though. Personally, I have replaced
UHF connectors with N connectors where I can, use nothing worse than
RG214 or LMR400 for interconnects at my station, and have the
interconnects as short as possible. I built many cables with exactly
the connectors and lengths needed so I avoid adapters. Everything
helps.

For field day I have a piece of 1 1/4 Heliax with N connectors to go
from inside the tent to the balun outside the tent, but I admit that
is just overdoing it!

73,

Mark
W7MLG

On Mon, Dec 28, 2015 at 7:04 PM, Ken Widelitz <[hidden email]> wrote:

> Hi Mark,
>
>
>
> Thank you for a cogent explanation. Clearly, I never thought about the math
> nor realized how much those little losses add up.
>
>
>
> 73, Ken, K6LA / VY2TT
>
>
>
>

Jim Brown-10

Re: P3 TX Mon Power Measurement Anomaly

On Mon,12/28/2015 6:40 PM, Mark Goldberg wrote:
> I have replaced
> UHF connectors with N connectors where I can, use nothing worse than
> RG214 or LMR400 for interconnects at my station, and have the
> interconnects as short as possible. I built many cables with exactly
> the connectors and lengths needed so I avoid adapters.

All of this is good EXCEPT that N connectors are NOT superior to good
quality UHF connectors (Amphenol 83-1SP) for use on the HF bands. Yes,
N-connectors are 50 ohm connectors, while UHF connectors are a bit off
of that, but it simply does not matter below 50 MHz, and it matters very
little below 200 MHz.

73, Jim K9YC

73, Jim K9YC
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Mike VE3YF

Re: P3 TX Mon Power Measurement Anomaly

In reply to this post by Ken Widelitz

Ken:

For what it's worth, I have noticed the same for years on my PowerMaster Meters that there is a bit of power drop when going thru BPF's.

In my case I use Dunestar 600's and I can see as much as 10w drop on the output of the BPF when the BPF is on and maybe a watt or so when the BPF is off. I always looked at it as this is normal. While it may be an anomaly, I wouldn't really loose any sleep as you still can drive the amp to full power.

I am not so sure that the Alpha Metering circuit LED's are all that accurate to measure the slight changes in Power In and Power Out.

73 De Mike
VE3YF

http://www.ve3yf.com
[http://www.ve3yf.com/]
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