Actually the fact that you were using the internal tuner doesn't change the chance that with the settings on the auto tuner don't tell you which direction the tuning of a certain antenna approach unity from. So there could be twice the variation allowed between the 2 antennas. So if one is coming from high impedance and stops at say 1.2:1 and the other is coming from low and stops at 1.2:1 then there is still a considerable difference between them. Putting an analyser on them to see the starting point the tuner is working on could explain a lot.
Jim Rhodes K0XU On Wed, Oct 23, 2019, 11:17 N4ZR <n...@comcast.net> wrote: > Thanks to everyone who replied, either directly or on the list. I'm > sorry, but I forgot to mention one key fact - I was running the amp's > ATU with both of these antennas. Assuming that the amp's power output > measurement is done at the input of the ATU and not the output, and that > the ATU settings for the two antennas ( ATU rRetune SWR and ATU Stop Tun > SWR ) are identical, this really does look to me like a difference in > amplifier gain rather than measurement error. > > The antennas are clearly a lot different - in fact the Windom will often > throw a Reflected Power fault when I first switch to it - but it seems > to me that after the ATU they should look the same. > > 73, Pete N4ZR > Check out the Reverse Beacon Network > at <http://reversebeacon.net>, now > spotting RTTY activity worldwide. > For spots, please use your favorite > "retail" DX cluster. > > On 10/23/2019 9:55 AM, Bob McGraw K4TAX wrote: > > Pete: > > > > The fact it requires more drive with one antenna as opposed to another > > says the two antennas do not have the same impedance. This is not at > > all unusual. Power measurements, with the methods used in ham radio > > applications, are based on voltage measurements with the impedance > > presumed to be 50 ohms. Ohms law says P = E²/R thus any change in R > > {impedance} will cause a change in Power indicated or measured at a > > given point. > > > > A second factor is SWR which is an indication of the relative > > impedance between the source {amp} and the load {antenna}. In this > > case, as example, a 1.5:1 SWR can be 75 ohms or it can be 33 ohms. > > SWR and Power meters are calibrated for 50 ohms and are based on > > voltage on the feed line. Again we see that the impedance or the R > > part being different will affect the Power. In order to deliver the > > same power into a 33 ohm load, the amp is required to deliver more > > current and thus more drive is required. > > > > From an RF measurement at a given point with different impedance's we > > find: > > > > 1500 watts into 75 ohms is 335 volts with a current of 4.47 amps > > > > 1500 watts into 50 ohms is 273 volts with a current of 5.47 amps > > > > 1500 watts into 33 ohms is 222 volts with a current of 6.74 amps > > > > From the above one can see the amp is required to deliver more current > > into a lower impedance and to do so will require more drive power. And > > from the above, one can see the voltage on the feed line is different > > with different loads. In this regard, in as much as we measure power > > as a voltage with a known resistance the power measurements can be in > > error. Our power indicating devices are calibrated for 50 ohms. > > Any deviation from 50 ohms will thus cause an error in power indication. > > > > A third component of SWR and Power measurements would be Common Mode > > Current on the feed line. This is usually current induced on the > > outside of the feed line from the power radiated from the antenna. > > This common mode current is not measured by our power indicating > > instruments. As a side note, Windom antennas are noted to exhibit > > high Common Mode Current conditions. The solution for CMC is to have > > a good Common Mode Choke at the feed point of the antenna, where the > > feed line connects, and also at the station end. > > > > Jim, K9YC, has and excellent paper on the topic: > > http://audiosystemsgroup.com/2018Cookbook.pdf > > > > Also, Rick, DJ0IP has a lot of valid information on his site: > > > > http://www.dj0ip.de/balun-stuff/ > > > > http://www.dj0ip.de/rf-cmc-chokes/ > > > > Now after all of this is said and done, I'd say your results with your > > KPA1500 are normal. Hence the reason for the variation in drive is > > due to the load impedance presented to the amp and the power > > measurement method. > > > > 73 > > > > Bob, K4TAX > > > > > > > > On 10/23/2019 6:59 AM, N4ZR wrote: > >> I have two 40M antennas, a Carolina Windom and a full-wave parasitic > >> sloper. I have just noticed that on any given frequency, my K3 must > >> deliver considerably more power on one than the other, in order for > >> the amp to produce 1500 watts. The difference is 27 watts on one vs > >> 42 watts on the other. Is this normal? > >> > > > > > > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:Elecraft@mailman.qth.net > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > Message delivered to jimk...@gmail.com ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
