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 91 
 on: October 10, 2019, 10:28:17 AM 
Started by KK4RF - Last post by KK4RF
Gentlemen:
     I am trying to build the 50 watt modulator shown in the '64 ARRL Handbook. The circuit shows a 6AV6 speech amplifier, then a 6CG7 dual-triode, the first part is the second stage of the speech amplifier, the other triode section is the driver, which then drives through a 1:3 interstage transformer (they call for a Triad A-31X), which then drives the modulator comprised of 4 1625 tubes, 2 each in parallel push-pull arrangement. I have the modulation  transformer called for, a Stancor A-3893. I have an interstage transformer, a Stancor A-4812, which has a primary that has a center-tap, apparently for a push-pull driver stage. The modulator driver stage has a single plate driving the transformer primary. I am not sure this will work or not with the interstage transformer that I have.
     Any ideas or insights will be greatly appreciated. This is going to be my main winter project, so I am trying to get all my ducks in a row, so to speak. Thanks in advance, 73s,  Marty Harpen, KK4RF, Suffolk, VA

 92 
 on: October 10, 2019, 09:35:17 AM 
Started by PA0NVD - Last post by PA0NVD
Mmmm interaction, a change in coupling between the two antennas... may be. Unfortunately that takes a lot of work to find out.
The two antennas are coupled with 12 inches of 50 Ohms twisted line in phase   The coax is connected to the 17 meter antenna, the 20 meter is at the end of the twisted line..
 The antenna is at full height now, 14,5 meters, and I am VERY reluctant to lower it again just to investigate, They work really nice.
I am a bit older Don, and I am happy I could get them up at all..  Grin.
When I was younger, I should have investigated and tried, but I think I leave it as is. It was an interesting effect, perhaps some day someone else will find the solution.
When time and energy come together, I will make a 1 GHz model and sweep it with the tracking generator and spectrum analyser using a reflection bridge. That gives the possibility to raise and lower by hand and see instantaneously what happens.

 93 
 on: October 10, 2019, 07:07:32 AM 
Started by W1RC - Last post by W1RC
We really need the attendance this time........This message posted by our good friend Bill, NE1B says it all.....

“You have probably heard that quote before!  Rain or Shine, this event needs YOU!  Without attendees, i.e. ticket sales, this will spell the doom of ham flea markets in New England.

If you have not had a chance to stop by HRO in Salem NH, you still can purchase "at the Gate".

Even with rain outside, there are still 4 Inside Building venues for visiting Vendors and Non-Profit Clubs.  Also many Flea Marketers with big tents.

Don't be a wuss.  Come on out Friday and Saturday.  Bring rain gear if it gets misty.  This is a great social event for the ham community!

NEDECN will have a table there both days in the Concessions Bldg.  Don't forget about the Tower Safety Class on Friday at noon by K1IR.”

73,
Bill, NE1B
--

 94 
 on: October 10, 2019, 03:01:05 AM 
Started by W4AMV - Last post by WZ1M
807 = 6L6 with a plate cap.
Regards,
Gary

 95 
 on: October 10, 2019, 01:00:21 AM 
Started by PA0NVD - Last post by KK4YY
I found this reference to the Moxon:

https://mirror.thelifeofkenneth.com/lib/electronics_archive/ATrulyPortableMoxonRectangleForNearlyNo-toolFieldAssembly.pdf

Quoting from that document:

"For 20 meters, the approximate minimum recommended height is about 3/8 wavelength or 26' above ground. At this height and above, the SWR curve will be very stable."

This would suggest that at a height of 8 meters and above there will be little change in SWR for a 20M Moxon. But, you saw a significant change going from 8 meters to 14 meters in height.

I still find no reference to the resonance moving lower as the height is raised. Perhaps there is some interaction of the two antennas being mounted together as you have done. If you're using separate feedlines try shorting the unused feedline and note any change in SWR on the active feedline to see if there is much cross coupling. Maybe there's a clue there.

 96 
 on: October 09, 2019, 08:13:28 PM 
Started by PA0NVD - Last post by PA0NVD
Quite a mystery for me as well Don. The coax ends in a choke of coax wound on a ferrite ring core and than directly connected to the antenna. The SWR is nice, with 1 kW input I have approx 4 Watts return. The mast structure is isolated from the antenna. But the antenna behaves like it should, front to back ok on both bands, better than 25 dB, also when it was longer and at low altitude. I get very nice reports, so there is gain as well.
When I raise the antenna there is a smooth decrease in frequency. At 6 meters altitude when cut to length the SWR was very good. Raised 2 meters more,and noticable worse SWR an a decrease in frequency of approx 30 kHz. (a little hard to measure) Raised to 14 meters again worse and a frequency decrease of approx 100 kHz. that is 0,7% too low So shorted DE and REFLECTOR  0,7 % and it was fine at 14 meters and approx 100 kHz high at 6 meters altitude. No mistakes there.

 97 
 on: October 09, 2019, 06:11:16 PM 
Started by PA0NVD - Last post by KK4YY
You've got a mystery on your hands, Nico. Antennas have been installed over every imaginable surface and yours, apparently, behaves differently than all the others. The good news is that it works!

I did some Internet searches looking for others who've experienced this effect and didn't find anything.

I wonder how you're feeding the antenna. Are you using a gamma match or directly connecting the coax? Is the tower/mast grounded at the antenna?

There's got to be an explanation to this.

 98 
 on: October 09, 2019, 04:48:58 PM 
Started by W8ACR - Last post by PA0NVD
You meant to say high IP3 I presume.

Indeed, Thats what I meant... Embarrassed

 99 
 on: October 09, 2019, 04:47:24 PM 
Started by PA0NVD - Last post by PA0NVD
Thanks for the input Don
I measure when the SWR is very low, so the cable length should not  be involved. I get a nice dip in SWR  I measure with a SWR bridge and run the frequency to find a SWR 1:1 or very close. Than I meausre FB in order to know that the antenne is behaving as should and that it is no false resonance. I rotate the antenne to see if reflection interact and point to a place without obstacles. It is on e push-up mast with Keenan guylines, so the lines do not interact. Always the same coax permanently connected.
The behaviour of the 20 meter antenna and the 17 meter antenna were the same, both went down in frequency when raised.
FB is very good so the reflector works as it should, both for 20 and for 17
The coax is attached with a good choke wound from coax at a ringcore. I measured balance (Rf voltage at the antenna wires near the input)and that's quite good.  Touching the coax firmly does not show any effect.
When raising the antenna I had to cut both the driven element as well as the reflector. I tried once without cutting the reflector, but than the SWR became high.
The only explication I can think off is that the soil is that conductive that is shorts the field lines A coil near a conducting surface gets less inductance and the frequency goes up. All quite unexpected and interesting

 100 
 on: October 09, 2019, 04:42:33 PM 
Started by W8ACR - Last post by W8ACR
Thanks again for the input. I think I have found the circuit that I will use. I came across the audio circuit used in the Collins 300G transmitter - Balanced transformer input to PP6J5's which are resistance coupled to 6A5's which drive class B modulator grids (810's). 6A5's are roughly equivalent to 6A3's (6A5's have indirect cathode heater). 6A3's are capable of 10-15 watts of audio power in AB1 depending on the plate voltage and type of bias used.

73, Ron W8ACR

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