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Re: sBitx antenna-first warning
John Cunliffe W7ZQ
Correct David.
How come so many still believe the myth that a bunch of passive elements ( antenna system) actually changes with power level? simple, they have a crappy SWR meter. It either has poor directivity, or the indication changes because of heating in the diode / changes in the detection circuit like core saturation at higher power levels.? |
Re: sBitx antenna-first warning
¿ªÔÆÌåÓýThis is not correct.? The output impedance of the PA does not change the SWR and resistance/reactance of the tuner/antenna combination.What one is really doing is tuning out the reactance of the antenna and setting the load resistance to the design value of the amplifier.? A reactive load increases the dissipation of the PA finals.? Tuning at low power within the voltage and dissipation limits is safe. David N1HAC On 8/3/22 3:44 AM, Ashhar Farhan via
groups.io wrote:
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Re: sBitx antenna-first warning
Please forgive my ignorance but I've never understood statements like this.? "That is because at lower power, the output impedance of the PA will be different." Since SSB is all about audio (no audio, no power) and the average power is about 40% of peak, what happens to the output impedance every time you speak into the microphone? Power is going to follow the audio and would be all over the place, from zero to peak power. How is this any different? Max KG4PID
On Wednesday, August 3, 2022, 02:45:06 AM CDT, Ashhar Farhan <farhanbox@...> wrote:
Many recommend tuning at a lower power. This risky as well. That is because at lower power, the output impedance of the PA will be different. Once you are tuned at QRP level, increasing the power will result in a mismatch again!? - f |
Re: Postman came with a box from hfsignals
Got it "turned ON" just for a moment before heading out to my teacher pre=planning day, today.? ?Used a safe 12V wall -based laptop power supply, will need to replace with connectors for a REAL power supply.? ?Very busy week for me with a lecture to give and many meetings and responsibilities so not? sure howmuch more I will be able to do.. But it turned ON and I got into the basic receiver/transceiver screen.? ?Hooray! Gordon On Wed, Aug 3, 2022 at 8:15 AM Don--AE4DW <AE4DW.Don@...> wrote: Same here..and of course I'm back traveling for an extended period starting next week, so its even more torturous that its not here by the weekend so I can at least takea peek. DHL has surprised me with early delivery a few times previous, so maybe... |
Re: Postman came with a box from hfsignals
Same here..and of course I'm back traveling for an extended period starting next week, so its even more torturous that its not here by the weekend so I can at least takea peek. DHL has surprised me with early delivery a few times previous, so maybe...
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On Wed, Aug 3, 2022 at 05:28 AM, Peter McCracken wrote:
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Re: sBitx antenna-first warning
"Many recommend tuning at a lower power. This risky as well. That is because at lower power, the output impedance of the PA will be different. Once you are tuned at QRP level, increasing the power will result in a mismatch again! "
Interesting. Every auto tuner manufacturer's instructions I have ever seen recommend that you tune at low power. I have a remote auto tuner at the base of a wire vertical that I tune on every band change. I have always tuned with low power using several different rigs over the decades and never noticed a change in SWR. I'll pay closer attention, but if I'm getting a change it has been to small to notice. Haven't received my sBitx yet, but it looks like there's room for a small board with SWR sensing and an absorptive bridge that you could manually switch in and out. If you wanted to get fancy you could route the measure to the Pi and integrate into the display. -- ? 73 ??? Bob? KD8CGH |
Re: A uBITX Woodie
#ubitxv6
Go retro and make a collage of the old logos from the past... Collins, Drake, Harmmarlaud, Hallicrafters, etc... be sure to add the builders kits like Heathkit and Eico On Wed, Aug 3, 2022 at 6:04 AM Alan Cooper <cqw7aln@...> wrote:
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Re: sBitx antenna-first warning
You can also get some TO-220 non-inductive resistors of a suitable
wattage, and mount those to the heatsink. 10-watt devices should be OK, and 50 ohms is close enough. Those devices aren't cheap, but they're effective. I use a 100-watt 50-ohm device mounted to a heatsink with an SO-239, and it's flat to well over 100 MHz. 73 Jim N6OTQ |
Re: sBitx antenna-first warning
You can control it in software, much simpler! The main reason for not putting in an RF sensor is that raspberry pi doesnt have an ADC On Wed, Aug 3, 2022, 4:21 PM Anthony Good <anthony.good@...> wrote:
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Re: A uBITX Woodie
#ubitxv6
?decorations: speaking of star war and star trek.... how about a layout of our solar system some where on the radio? Alan? w7aln On Tue, Aug 2, 2022, 10:35 PM Thomas White AD6TW <tawhitejr@...> wrote:
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Re: sBitx antenna-first warning
Anthony Good
I'm wondering if we could add an SWR bridge and use the reverse sense voltage to automatically fold back the RF power?? Perhaps control the gain of Q11 to accomplish this? (I have an sBitx on order...) 73 Gody K3NG On Wed, Aug 3, 2022 at 6:15 AM Ashhar Farhan <farhanbox@...> wrote:
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Re: sBitx antenna-first warning
¿ªÔÆÌåÓý?Scott, where did you get three legs? ?If all the resistors are parallel there will be only two legs. ?Connect one to the ground of your transmitter and one to the antenna pin in your output connector. ?A dummy load connect just like an antenna. ?It replaces the antenna for testing purposes. All my dummy loads have a BNC connector and I connect them to the BNC out connector of my transceivers. ? Google dummy loads¡. ?You will find dozens of them and check YouTube for videos. Dave K8WPE since 1960 David J. Wilcox¡¯s iPad On Aug 3, 2022, at 6:07 AM, Scott KE8KYP <scott_massey@...> wrote:
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Re: sBitx antenna-first warning
Scot, You have to build a resistive bridge like this : Here, each of the 50 ohm resistors should be able to dissipate 10 watts. So, you make a 10 watt resistor by paralleling 10 resistors of 470 ohms each.? - f On Wed, Aug 3, 2022, 3:37 PM Scott KE8KYP <scott_massey@...> wrote: Hi Ashhar, |
Re: A short video of sBitx working
I understand the need for a low pass filter. That is needed for the IC 7300 also which I built a low pass filter for it which was easy enough. It produces real nice harmonic spurs right through the AM? broadcast band without one, and gives a normal watt meter fits because one cannot tell where the rf is getting produced is in the electromagnetic spectrum. It also causes an swr meter on the radio to read incorrectly since the reflected spurs power causes the rf foldback to activate and cut the transmit power way down. the rf tranformer in the sBitx has me wondering... is the what would pass for an output matching network in the PA, that is being refered to ? thanks Alan On Tue, Aug 2, 2022, 6:09 PM Ashhar Farhan <farhanbox@...> wrote:
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Re: Postman came with a box from hfsignals
I received notification from DHL today with delivery scheduled for Monday (not bad since I am in Ireland) , obviously the basic kit has started to ship, will look forward to much experimenting with this :) On Tue, 2 Aug 2022 at 23:20, Shirley Dulcey KE1L <mark@...> wrote:
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Re: sBitx antenna-first warning
Many recommend tuning at a lower power. This risky as well. That is because at lower power, the output impedance of the PA will be different. Once you are tuned at QRP level, increasing the power will result in a mismatch again!? - f On Wed, Aug 3, 2022, 12:52 PM Ashhar Farhan via <farhanbox=[email protected]> wrote:
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