| Author |
Message |
    
Lester_elm
Member
Username: Lester_elm
Post Number: 53
Registered: 12-2007
| | Posted on Sunday, May 30, 2010 - 1:57 am: | 
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I have a PARADYNAMICS PDC-700 meter. The specs for this meter state that it is designed for a frequency range of 25 - 30 MHz....
My question is: How can this meter be modified to cover a frequency range of 3.5 - 30 MHZ???
Please be specific in the details....Not just "swap out some components".....detail what components need to be replaced, removed, or added.
Any help would be greatly appreciated.
CEF #964 HAM #276
Let's play radio!
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Tech833
Intermediate Member
Username: Tech833
Post Number: 190
Registered: 12-2001
| | Posted on Sunday, May 30, 2010 - 12:54 pm: |
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No modification necessary. The meter will still indicate at lower frequencies. However, the accuracy will suffer. What you need to do is compare the readings at various frequencies with a known accurate meter, and create a calibration graph for your PDC-700. Then, you use the readings from your meter and either add or subtract the calibration % for the frequency range you are using.
It's worth noting that NO meter will cover that kind of range with 100% accurate readings. That is why even the popular Bird meters have slugs for different bands. And, the HF slugs usually come with a calibration graph.
Your radio 'Mythbuster' since 1998
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Lester_elm
Member
Username: Lester_elm
Post Number: 57
Registered: 12-2007
| | Posted on Sunday, May 30, 2010 - 2:49 pm: |
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Paul, I do understand what you are saying.
However, I would like for the meter to be at least close to the actual power output. As it is now, 100W carrier output into a 50W 300W dummy load (as measured on an oscilloscope) results in the following readings on the PDC-700:
FREQ (MHz) - PWR (W) - SWR
30.000 ------- 110 ----- 1.2
29.000 ------- 105 ----- 1.2
28.000 ------- 100 ----- 1.1
24.500 ------- 98 ------ 1.1
21.000 ------- 98 ------ 1.1
18.000 ------- 74 ------ 1.0
14.000 ------- 57 ------ 1.0
10.000 ------- 38 ------ 1.1
7.000 -------- 22 ------ 1.1
3.500 -------- 6 ------- 1.1
1.800 -------- 2 ------- 1.2
I know that I am not going to get 100% accuracy with this meter but I would like to get at least within 20% across the board from 3.500MHz to 29.000MHz. Radio Shack, MFJ, and others market fairly inexpensive meters that accomplish this. I would like to be able to mod my PDC-700 to do the same thing. SWR portion of the meter works fine with accurate readings across all HF Ham bands.
What is it in the watt meter that limits its' use to 25MHz - 30MHz?
CEF #964 HAM #276
Let's play radio!
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Tech833
Intermediate Member
Username: Tech833
Post Number: 192
Registered: 12-2001
| | Posted on Monday, May 31, 2010 - 1:16 am: |
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If the sample came directly from the diodes on the striplines, then frequency would be of little calibration concern. The diodes would work well up to their design frequency and then fall off over that.
From what I can tell, that circuit uses other components, and the LC or whatever network used to bump up the sample to a high enough current to deflect that meter are the issue.
It appears (from your excellent data) that it is fairly useable down to about 17m band. That isn't too bad at all. The big problem is on 80m and 160m! Admittedly, those are the two bands most problematic for even the amateur meters. The MFJ meters either don't work on 160, or are so far off, they may as well not work there. I gave away mine because it didn't even work well on 80m (my favorite band) like their ads says it does.
Best bet- Find an old Pace or similar SWR/Power meter like they used to sell 20 years ago+. I have one and it is really accurate from AM BCB all the way to 6m. Nothing but 2 diodes and a cap in there. No frequency determining parts. Copy that schematic for your PDC and see if it provides enough current from the untuned sample to deflect the PDC meter enough to satisfy.
Your radio 'Mythbuster' since 1998
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Geekster
Intermediate Member
Username: Geekster
Post Number: 195
Registered: 6-2004
| | Posted on Monday, January 17, 2011 - 7:21 am: |
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If this PDC 700 was built in the 90's I would say that this would be a daunting task. The insertion loss was unsatisfactory below 20Mhz. Back in the 90's the diodes used were 1N4148 and the construction of the coupler didn't really allow for much play.
It could be possible but you may consider swapping the diodes for something like 1N270's or better yet modify the coupler. |
American_radio_18
New member
Username: American_radio_18
Post Number: 5
Registered: 2-2011
| | Posted on Sunday, June 05, 2011 - 9:30 pm: |
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I have a TR-1000 mobile dosy, and an RCI-2970N2 My question is why is the meter reading less than what the radio is rated for PEP? The radio is said to be 200 SSB and its reading about 150 PEP |
Charliebrown
Intermediate Member
Username: Charliebrown
Post Number: 206
Registered: 10-2002
| | Posted on Sunday, June 26, 2011 - 12:07 pm: |
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I HAVE TWO DOSY METER'S. ONE IS THE 3001 AND THE OTHER IS 4001. THE DOSY 3001 WILl SHOW A 1.3 SWR AND THE DOSY 4001 WILL SHOW A 1.8 SWR. BOTH METER'S ARE CONNECTED TO THE SAME COAX ,RADIO AND ANTENNA. WHY IS THERE SO MUCH DIFFERENCE IN THE SWR READING AND HOW CAN ONE TELL WHICH IS THE ACCURATE READING? ALSO WHAT METER DOES ANY TECH OR ANYONE ELSE WOULD SAY TO BE MORE ACCURATE THAN THESE? |
