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Author Topic: 2Meter AM  (Read 39853 times)
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WU2D
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CW is just a narrower version of AM


« Reply #50 on: April 03, 2010, 09:13:08 AM »

The 2M station is working nicely but my 20 Watts out of the SCR-522 TX is being heard better than I am hearing everybody else. The little R-508 command set RX is just too noisy. I measured around 1 - 2 uV sensitivity with my old HP608. I can tell that stations are in there but I can not quite copy the outlying stations. When they turn the beam right on me, I hear them.

I am thinking of strapping a preamplifier on the front. I had an old 2M 40673 dual gate mosfet preamp I built many moons ago and simply strapped that in front. Result - very little improvement. The modest noise figure and gain of the mosfet was not enough to set the noise figure of the system and overcome three cascaded pentodes worth of noise!

I could build a better more modern preamplifier with a lower noise figure and more gain. Or I could possibly drop out the two RF amplifier stages and convert them to Fets and run them on the +12V fil voltage.

Ideas?

Mike WU2D
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KM1H
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« Reply #51 on: April 03, 2010, 11:02:30 AM »

Most flea power 2M rigs are far from 100% modulation, never mind having any enhancements, which makes it hard to copy.

Id opt for FET's in what you have, external additions will overload an already marginal radio. Are you going to have gain control on them?

Carl

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WU2D
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CW is just a narrower version of AM


« Reply #52 on: April 03, 2010, 05:04:40 PM »

I have improved the AGC system in the original radio which consisted of a diode section on the last IF stage and a diode section on the detector first audio tube which is separate from the detector. Never heard of two AGC detectors on separate stages both contributing to the AGC line. One seemed to be for the IF stages and one was primarily to control the two RF stages which are not remote cutoff types. In any case the AGC was weak at best. So I ripped it out and put in a schottky detector and fed the AGC line and re-biased the set point on the RF stages and now it works sweetly. 

Yes the stages will have to be controlled if I go solid state.

Mike WU2D


* R13B_AGC_MOd.jpg (331.6 KB, 2755x1854 - viewed 660 times.)
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K1ZJH
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« Reply #53 on: April 03, 2010, 05:10:44 PM »

I am thinking of strapping a preamplifier on the front. I had an old 2M 40673 dual gate mosfet preamp I built many moons ago and simply strapped that in front. Result - very little improvement. The modest noise figure and gain of the mosfet was not enough to set the noise figure of the system and overcome three cascaded pentodes worth of noise!

I could build a better more modern preamplifier with a lower noise figure and more gain. Or I could possibly drop out the two RF amplifier stages and convert them to Fets and run them on the +12V fil voltage.

Ideas?

Mike WU2D

You'd be far better off dropping a few stages of gain ahead of the first mixer, and let the first stage (your mosfet preamp) set the noise figure for the receiver. None of those early tube VHF RFA stages are worth the powder to blow them to heck these days.  Another important question, what is the IF bandwidth?  If it is excessive that won't help with weaker signal reception either.

Pete k1zjh
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WU2D
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CW is just a narrower version of AM


« Reply #54 on: April 03, 2010, 05:16:55 PM »

Here is the front end showing the 9003 pentodes and the 200V zener regulation I put on the front end stages (mostly for the oscillator) to stabilize it. The IF bandwidth is excellent, around 8 kHz.


* R13B_SCH_FrontMM.jpg (572.01 KB, 3485x2058 - viewed 657 times.)
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Pete, WA2CWA
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« Reply #55 on: April 05, 2010, 01:58:47 PM »

I know of few guys who sometimes use 2 merer AM as sort of an intercom, but, for me, 2 meter AM has about as much appeal as warts. I suspect many 2 meter AM rig owners find the rig(s) sitting on a shelf collecting dust. I got rid of most of mine years ago.
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Pete, WA2CWA - "A Cluttered Desk is a Sign of Genius"
Pete, WA2CWA
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« Reply #56 on: April 05, 2010, 05:28:49 PM »

I know of few guys who sometimes use 2 merer AM as sort of an intercom, but, for me, 2 meter AM has about as much appeal as warts. I suspect many 2 meter AM rig owners find the rig(s) sitting on a shelf collecting dust. I got rid of most of mine years ago.

That's how I feel about operating 2 meters today in general on any mode for that matter. I guess if you had a few buddies and you all lived in the same town AM would be fun to run if it was still the old tube stuff.

6 meters is really the way to go for AM on VHF and you'll easily get cross country contacts. You need a good directional antenna though like a yagi which has some gain factor to it. I originally used a dipole and then went to a 3 element. Night and day by comparison.

During the last secondary peak, I remember checking into two different AM nets on two different openings in the CA area. Also worked England on 6 meter AM during that time.
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Pete, WA2CWA - "A Cluttered Desk is a Sign of Genius"
ke7trp
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« Reply #57 on: April 06, 2010, 10:49:27 AM »

We have a group of around 10 people here.  Some are on HB rigs, Some on clegg, some on heath. 

I can see one real flaw with 2 meter am.  Lots of the guys use solid state icoms or yaesus.  The receiver is narrow and they complain if you come on of Freq. They have an SSB mentality. 

To me.. You cant be off freq on a boatanchor.. tune me in damn it.

C
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Pete, WA2CWA
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« Reply #58 on: April 06, 2010, 01:22:58 PM »

We have a group of around 10 people here.  Some are on HB rigs, Some on clegg, some on heath. 

I can see one real flaw with 2 meter am.  Lots of the guys use solid state icoms or yaesus.  The receiver is narrow and they complain if you come on of Freq. They have an SSB mentality. 

To me.. You cant be off freq on a boatanchor.. tune me in damn it.

C

With the proliferation of new satellite and digital type activities now appearing more regularity on 2 meters, having a "SSB mentality" is probably not a bad thing. "Wide as a barn door" or having a rig that drifts around the band, probably won't gather you many friends on 2 meters.
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ke7trp
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« Reply #59 on: April 06, 2010, 02:30:53 PM »

We have one Freq that is permitted. 144.450.  The rest of the band is tightly controlled in the valley here. 

C
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WU2D
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CW is just a narrower version of AM


« Reply #60 on: April 08, 2010, 08:22:56 PM »

Just for fun I borrowed an LNA and stuck it in front of the R-508 noisemaker receiver and tried it tonight. It was an LNA-3000 from RF-Bay.

http://cgi.ebay.com/40-3000MHz-5V-Low-Noise-Amplifier-LNA-3000B-New-SMA_W0QQitemZ400079895151QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item5d269eba6f#ht_500wt_975

Wow is all I can say. Even with no filter inline, it drastically pepped up the front sensitivity. It has a low enough noise figure and high enough gain to overcome the two pentodes. The AGC works beautifully too and I have not any sign of overload even with aircraft in the 136 MHz range. That is one nice amplifier. Alas it goes back to work tomorrow but I got the message. With a low enough NF and enough gain, all I need is an outboard amp.

Mike WU2D
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KM1H
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« Reply #61 on: April 08, 2010, 09:20:56 PM »

Be happy that old tube rig isnt bothered by input VSWR.  That looks like an instrumentation amp or something for one of the Icokenseu wide band receivers that all varieties of techs and engineers run around with.

You could build a 2M only version for about a tenth the price if youre serious about pepping up that thing

Carl
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WU2D
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CW is just a narrower version of AM


« Reply #62 on: April 11, 2010, 02:53:24 PM »

It is quite shocking to see what they have been able to do with the wideband MMICs. The gain and noise figures are approaching what some of the best narrowband LNAs of the 1970's could do. The difference is that they are doing it over such a wide band and the match on both input and output is darned good and the IP3 is impressive.

Mike WU2D
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W2PFY
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« Reply #63 on: April 13, 2010, 02:28:08 PM »

Quote
We have one Freq that is permitted. 144.450.  The rest of the band is tightly controlled in the valley here.

What happens if you run Am anywhere you want or SSB or FM for that matter?

In the Albany NY area, perhaps we don't have as many repeaters but I think so long as your not operating on an input or output frequency, there shouldn't be a problem? Am I wrong? Just now getting back into 2 Meter AM.

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ke7trp
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« Reply #64 on: April 13, 2010, 03:30:22 PM »

ITs a gentlemans agreement. As long as your not bothering a repeater your prob fine.

C
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KM1H
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« Reply #65 on: April 13, 2010, 04:26:47 PM »

AM runs below the FM stuff, no one will hear you up in that area but it could be fine for semi private QSO's on dead frequencies.  The problem is the dweebs running the APRS and other pointy head stuff Grin down around 144.4. They think they own wherever they plop. There should be plenty of room in the 144.4 to 144.6 area, just listen around during peak hours.

Carl
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« Reply #66 on: April 14, 2010, 12:28:43 PM »

Not hear.. 2 meter is a very active band.

C
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« Reply #67 on: December 01, 2013, 02:50:47 PM »

 Smiley Yes, I agree too. 50.400 is a great place to to do AM !

<snip>

6 meters is really the way to go for AM on VHF and you'll easily get cross country contacts. You need a good directional antenna though like a yagi which has some gain factor to it. I originally used a dipole and then went to a 3 element. Night and day by comparison.
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Stephanie WX3K
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"Thunder is good; Thunder is impressive but it is lightning that does the work" ...Mark Twain
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