Topics

Erratic Readings

Daniel Walter
 

Having a lot of trouble finding useful readings. 

Have FDIM unit that has had h/w modified to v 2.1 per Farhan's suggestions
Firmware 2.1 uploaded to unit. Calibration redone.
It appeared to work well at first- curve and SWR readings on a real 7MHz antenna corresponded to RigExpert readings.

However, since then, readings have been very erratic. SWR with open circuit reads either 99.9 or 17.4- nothing else. (They were pretty erratic before the upgrade, too.)
Putting a 51ohm 5% metal oxide resistor  (51ohms on DVM) on Antuino gives 1.1 at 1 MHz, 1.7 at 4 MHz, 3.0 at 8 MHz, 3.5 or 4.0 SWR at 10 MHz, 5.8 at 14 MHz and 8.7 at 20 MHz.
A known 1.1:1 antenna at 28 MHz shows 99.9 on the Antuino. A good 50ohm dummy load shows 3.5 across the spectrum. Most real antennas show 99.9:1 or 17.4:1 at any frequency.

I have checked all solder joints- none seemed bad, but touched up many of them anyway. Did not make any difference. Looked for solder bridges and saw none.

Any ideas?

Dan,  NM3A 

Daniel Walter
 

Update
Known good antenna at 28 MHz DOES show 1.0:1- I was going thru antenna match. My bad.
Long leads  (20") on 51ohm resistors, may account for readings.
Still doing the 99.9/17.4 tho. 

Dan  NM3A

Daniel Walter
 

Further update

Seems to work well when open reading is 99.9, but not at all when open reading is 17.4.

Dan  NM3A

 

Dan,

I have posted the lookup table below: the first column has t be divided by 10.
there are no values between 99.9 and 17.4.

I have proposed a mod to Farhan to calculate in 0.2db and then lookup table
with smaller steps. Mostly we are interested between swr 1-3 ..

Do an open cal and try rotate the knob. After that power cycle. See if that
improves the situation. My first test unit is a bit flakey the others are fine.

Raj

999, // 0 db
174, // 1 db
87, // 2 db
58, // 3 db
40, // 4 db
35, // 5 db
30, // 6 db
26, // 7 db
20, // 8 db
19, // 9 db
18, // 10 db
17, // 11 db
16, // 12 db
15, // 13 db
14, // 14 db
14, // 15 db
14, // 16 db
13, // 17 db
13, // 18 db
12, // 19 db
12, // 20 db
12, // 21 db
12, // 22 db
11, // 23 db
11, // 24 db
11, // 25 db
11, // 26 db
10, // 27 db
10, // 28 db
10, // 29 db
10 //30 db

At 24-10-19, you wrote:
Update
Known good antenna at 28 MHz DOES show 1.0:1- I was going thru antenna match. My bad.
Long leads (20") on 51ohm resistors, may account for readings.
Still doing the 99.9/17.4 tho.

Dan NM3A

Michael Shreeve
 
Edited

Just got one, checked and it was modified correctly. Haven't ran it at all yet. At least I won't need to do the mod.It has 2.1 in it.

Michael Shreeve
 

I would assume , since your posting after this date, that your's has the 2.1 update , and has the mod ? I wouldn't take apart to check as its kind of a pain to get it back together correctly. I think you can tell you've got the mod if you have a first screen showing -105 dbm rather than the first screen in the 90s like many I've seen.