The python code of my previous post works, but the first SWR computation is rather silly.
I started with a formula for SWR given forward and reverse power, computed?power from the voltages.?
Much easier to just use the voltages.

So replace this python code:

Here's the webpage from which the formulas for SWR from voltage and power readings came:
? ??

Formulas for computing s11 (the reflection coefficient) from antenna impedance, and then SWR from s11, are found here:
? ??

Jerry, KE7ER?

Attached is the schematic from? ??

Half of the transmitter's output voltage appears?between resistors R3 and R5.?
R1 and the antenna form the other half of the bridge.
D1 is a simple detector, it charges C1 with the peak voltage from that signal,
so the meter sees half of the peak voltage from the transmitter.

How D2 and C2 work is not so obvious, but they?detect the peak of the
instantaneous difference in voltages across the bridge.? Those AC voltages can
be up to 90 degrees out of phase if the antenna is purely inductive?or capacitive.
In that part of the AC cycle where the left side of the bridge is more positive
than the right side, current can flow through D2, charging cap C2.
So the node between D2 and C2 has an AC component equal to the RF antenna
voltage on the far side of C2, plus the DC component that C2 was charged with.
The LPF of R8 and C4 removes the AC component so the right side meter
can measure the peak of the difference voltage.

For you python fans, here's my python code.
Both print statements give exactly?the same value of?3.0170334337197553
This code uses python's complex number notation, the abs() function
returns the amplitude of a complex value.

##################################


Note that transmitter power can be calculated from the left meter reading Vm1:
? ? Vrms =? (2*Vm1)/sqrt(2);? ?Power = Vrms*Vrms/50?
This assumes that the antenna is replaced by a fourth 50 ohm resistor.
A single DVM can be used to check the voltage at each meter position,
making this an extremely cheap way to measure SWR.

Most resistive SWR bridges only light up an LED, and you tune the?
antenna for minimum brightness.? This can achieve a reasonably low SWR,
but doesn't actually measure anything.

Jerry, KE7ER

Ah, sounds right.
He might try moving the 16mhz resonator on his Nano
instead of going to the bother of building a new filter.

Jerry, KE7ER

What complaint is Gerard trying to fix?
From this thread, I get the impression that he wants to move to an 11.059mhz IF so he can use
newer firmware that was written for V5 and V6.
? ??/g/BITX20/topic/77892249

Moving to different IF crystals could be fun and educational, you will want a way
to measure the passband of the completed filter and evaluate it for shape and attenuation.
Modifying the source code of the new firmware for 12mhz would likely be easier.

We had a few v3 and v4 uBitx users complaining of a strong audio hetrodyne anywhere they tuned, on all bands.
? ? /g/BITX20/message/40038
It was determined that the fourth harmonic of the 12mhz BFO was beating against the third harmonic
of the 16mhz processor clock.? The Nano on the Raduino has a 16mhz resonator, not a quartz crystal.
The resonator can vary by 100khz, so only a few users were unlucky enough to have it fall into
the 12mhz passband.
The solution for v3 and v4 owners was to swap out the Nano, or somehow
push the 16mhz resonator over a few khz.
The permanent solution on v5 and v6 was to move the IF from 12mhz to 11.058mhz.

The transmit spurs on the 15, 12, and 10 meter bands were due to
distortion in the 45mhz amp creating a 90mhz second harmonic?
that then mixed with CLK#2 to create a below 30mhz product.
For example, when transmitting on 21mhz, CLK#2 is at 45+21=66mhz.
And the 90mhz harmonic of 45mhz mixes with 66mhz to create a spur at 24mhz.
These spurs were suppressed on v5 and v6 by cleaning up the 90mhz harmonic.
? ??/g/BITX20/topic/28558651#62226

Yes the 15,12,10 meter bands are not working very well for us at?
this stage of the sunspot cycle.

Jerry, KE7ER

Curt,

on another thread Gerard had found a 16khz signal on the audio output of his rig with the 12MHz SSB filter. I went back and did the same test on my v4 board and found the same thing. I then checked my v5, it did not have the 16khz signal on it.?

That is one of the reasons that he is looking to swap out the 12MHz for 11.059MHz filter.?

73
Evan
AC9TU

The discussion on dummy loads got me thinking of resistive SWR bridges.

Would be good to include an SWR meter as part of a basic uBitx test set.
Here's a resistive bridge that I like:? ??
The two meters could be a single DVM that switches between the two positions.
They measure half of the voltage from the transmitter, also the voltage across the bridge.
I've written some python code given the two voltage readings, it can indeed compute
an SWR that agrees with an SWR calculated only from the assumed complex impedance
of the antenna.? A fourth 50 ohm resistor at the antenna port makes this a 8W dummy load.
From the forward voltage reading, we can get an accurate reading of transmitter power.

In quantity on mouser, 50 ohm 2W resistors are $0.05 each,?
and 1n5711 diodes are $0.10 each.? Switching it in and out could be done
with jumpers on test pins.? The other parts are all a penny or two in quantity.
So we're at a total of about $0.50 in parts for an instrument that provides
a dummy load, an SWR meter, and measures transmitter power.
The remainder of the test set might be just some resistive attenuators?
and a diode RF probe.? So a total parts cost of $1 is not out of the question.

I believe the uBitx receiver can deal with an incoming signal approaching 0dBm,
A 0dBm signal is 0.224 volts rms, or 0.316 volts peak.
With microamp level diode currents, a shottky diode should conduct enough to make
measurements down below a 0.1 volt forward voltage, such low level measurements?
will only be accurate if corrected through the use of a calibration chart.

So a diode RF probe might be sufficient, though an AD8307 would be better.
AD8307's can be had for down around $0.30 each on eBay.

A Nano clone such as used on the Raduino should be able to send
at least a 2mhz and perhaps a 4mhz square wave out an IO pin.? A few resistors
could reduce this down to a calibrated 0dBm signal for injection into the receiver.
No need for an extra Si5351 output as a signal generator to drive the receiver
antenna port when doing a receiver check out.
Verifying that the 30mhz LPF and first mixer work properly across all bands
could be done?when checking out the transmitter in CW mode.

A basic test set such as described above is down around $1 total in parts.
A complete set of step by step instructions should be included.
It would be of considerable help to the many uBitx owners that come into the forum
wondering how to proceed when their rig doesn't work.
Having one well defined method of doing the measurements would make
troubleshooting via forum posts much easier.

Jerry, KE7ER

Gerard

you won't notice much difference on receive if you move the xtal frequency.? this change is to reduce the transmit spurious on certain bands.? and these bands are higher in the spectrum, and not working very well now.? the designer noted the impact of this change when the v5 was released - I suggest reviewing his notes to see if it is worth the effort for your use.?

73 curt

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If the SWR doesn't change much for your resistor(s) when you dunk them
in the motor oil,?it should be fine.? ?Worth experimenting.

My primary concern would be that motor oil is much more objectionable if it leaks
than linseed oil or mineral oil.? Perhaps best suited for a radio station
that happens to be in the garage.

We should note, this is not needed for? a uBitx, with 10 Watts output.
On a uBitx, probably best to just use air cooled resistors.
Bothering with an oil bath to dissipate heat starts making sense at 100 Watts or more.

Jerry, KE7ER

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Hello,

If you use the Linseed oil since it will with heat go "rancid" with use.
The better solution is to use the new ALL Synthetic motor oil, much better since it was designed for use in "High" Temperatures.
73's
Steve
AB2ET/4

IS not for you

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e daie rompicoiota !!!!
smetti di fare il bovaro quando parli di questi argomenti !
dai consigli corretti per far crescere le persone, non per farne dei cacciavitari !
se vuoi dei buffoni al posto di OM va a dare consigli altrove !

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