#659

Over the years I built several evolutions of the TT2 transmitters. The 40th Anniversary goes together very quick once I got my cataract surgery & 5x jewelers magnifier.

My uncalibrated Fluke 79 III agrees with my Harbor Freight Cen-Tech & very close to my Lowes cheapie (the one I loan out when needed). The point is my measurements are anywhere from 5-20% accurate, but what really counts is my on the air reports which is how I test my antennas once I lay down my Rig Expert antenna analyzer to get real world results.

*** Funny note, all three of my VOMs tell me my commercial 20w 50 ohm dummy load is 53.9 ohms, while Rex's Tuna Helper ][+ T/R switch 50 ohm dummy load measures 49.6 ohms. It makes a 14 Hz difference in the transmit frequency of my TT2 at 7.030 MHz, so if you are using a very long wire antenna swinging in the breeze suspended from a tree or carbon fiber mast you may incur slight differences in transmit frequency shifts. I mention that only as I occasionally see that happening on homebrew QRPp radios that are out doing POTA activations & now can say I probably know it's due to changing antenna impedance in the wind more that the previous thought I had of it being due to changing supply voltages.

My 7.030 crystal measures 7.029184 according to my Icom R-75 VFO ears into a 49.6 ohm dummy load, 7.029126 at 53.9 ohms. The TX is very stable into a dummy load.?

The peak-n-hold circuit of my Tuna Helper measures 271mw output at 12.1 volts supply voltage. Changing the supply to 13.2 volts only netted 313mw output.?

The Tuna can on the right is my 40th anniversary TT2 TX with 40m circuitry.?

The can on the left is my Tuna Helper ][+ RF sensing T/R switch with built-in 50 ohm dummy load.?

Tomorrow later afternoon I will throw up my 49:1 QRP EFHW antenna to see if I can scrape up a few signal reports from Winter Haven, FL. This weekend is Winter Field Day, but due to my extended & beloved wife care activities I can only dabble here & there before she yells for me on .52 with her Yaesu FT-25 HT, lol.

More testing to come, I look forward to the Buildathon.

Cheers,

Davey --KU9L & Sheila --KB9YYI?

#660

The Tuna Helper T/R Switch has a peek-n-hold circuit at the dummy load, I forgot to add in the voltage drop of the 1N5711 diode for better accuracy. The data sheet says:?1N5711 Diode. This Schottky diode is used in amplifier wattmeters for the characteristic of a low forward voltage drop. 70V 15mA, Forward Voltage Drop: 0.41V at 1mA

So I need to add about 0.41 volts to my voltage measurement in the calculation:?Peak Envelope Power is usually measured in a resistive load, and in that case, it is simply given by?PEP=V²/2/R?(where V is the peak RF voltage) or PEP=V_RMS²/R.

So in my case, I measured 5.21 volts + 0.41 volts = 5.62 volts. 5.62 volts squared = 31.5844. 31.5844/2=15.7922 15.7922/49.6 (D.L. Resistance in Ohms) = 318mw at a supply voltage of 12.1 volts.

A supply of 13.2 volts therefore yields ~367mw. In reality, the supply voltage difference will make no real difference in the received signal on the other end.

So, what can be worked with 300 and 2/3 mw of power? In 1987 I casually worked all 50 states on 40 and 15 meters with 1-watt output from my Heathkit HW-9 CW QRP rig into a 40M rotatable Dipole mounted up at 50 feet and a 3 ele yagi for 15 M mounted at 50 feet. Like anything else, getting the antenna as high as possible was like adding a 10-watt amplifier to my signal from N. IL. If you grab several different 40M crystals for the popular POTA frequencies the contacts will be made. QRP takes skill and patience on both ends, but I am very impressed with the POTA folks who tolerate QRP VERY WELL. In 1987 I was accused of making the QRO operator's life a living Hell, LOL.

Cheers,

Davey --KU9L & Sheila --KB9YYI

#661

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Davey's right. It can be done. Patience and persistence are key factors. I've made a number of contacts (including QRP-QRP) at the 1/4W on level using low (18ft) dipoles on 40m and a few on 20m under similar conditions. 40m QSOs were TT2.?

One TT2 contact was with a QCX out near Phoenix, if I remember correctly, also with a less-than-stellar antenna. I'm in the Joplin, MO area.

I find that at around 2W-5W, one often has a really reasonably good shot at working loud stations. ?In fact, I prefer about 1W-2W for casual QRP. DX is quite possible.

It's a real kick to see where your TT2 or Rockmite signal goes on the RBN. You might be surprised.

-HRS

H. Russell Smith, N0QLT

Show quoted text

On Jan 26, 2024, at 9:23?PM, David Knapp via groups.io <renewables@...> wrote:

?

The Tuna Helper T/R Switch has a peek-n-hold circuit at the dummy load, I forgot to add in the voltage drop of the 1N5711 diode for better accuracy. The data sheet says:?1N5711 Diode. This Schottky diode is used in amplifier wattmeters for the characteristic of a low forward voltage drop. 70V 15mA, Forward Voltage Drop: 0.41V at 1mA

So I need to add about 0.41 volts to my voltage measurement in the calculation:?Peak Envelope Power is usually measured in a resistive load, and in that case, it is simply given by?PEP=V²/2/R?(where V is the peak RF voltage) or PEP=V_RMS²/R.

So in my case, I measured 5.21 volts + 0.41 volts = 5.62 volts. 5.62 volts squared = 31.5844. 31.5844/2=15.7922 15.7922/49.6 (D.L. Resistance in Ohms) = 318mw at a supply voltage of 12.1 volts.

A supply of 13.2 volts therefore yields ~367mw. In reality, the supply voltage difference will make no real difference in the received signal on the other end.

So, what can be worked with 300 and 2/3 mw of power? In 1987 I casually worked all 50 states on 40 and 15 meters with 1-watt output from my Heathkit HW-9 CW QRP rig into a 40M rotatable Dipole mounted up at 50 feet and a 3 ele yagi for 15 M mounted at 50 feet. Like anything else, getting the antenna as high as possible was like adding a 10-watt amplifier to my signal from N. IL. If you grab several different 40M crystals for the popular POTA frequencies the contacts will be made. QRP takes skill and patience on both ends, but I am very impressed with the POTA folks who tolerate QRP VERY WELL. In 1987 I was accused of making the QRO operator's life a living Hell, LOL.

Cheers,

Davey --KU9L & Sheila --KB9YYI

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Power vs. Supply Voltage comparison

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