Today was a bit of a strange day. Spent some time:
- Constructing some receiver antennas
- Sorted out some 50Ω Dummy loads
- Tested charged and discharging a 4AH; 3S battery
- Realized I need to build an electronic Dummy load
=Antenna testing photos= (excuse the mess!)
Built a pair of 70CM Dipole antennas both tuned fairly well; nice and straight forward to build + tune (make them too long + trim!) Both are modestly low SWR (~1.35:1), and close-enough to 50Ω for my receive only purposes.
Experimented with some multi-element array setups; but without having the space to really clear the immediate area I’m seeing all sorts of intermittent and unexpected behaviour that I’m having to chock up further work on multi-element. Would love to get working on some low-gain Yagi’s, but they’ll have to wait!
My Test setup is FAR from ideal. I’ve got countless things within range of the antennas I’m testing causing reflections and nulls that don’t exist. Without space to really TEST the things; I’m not going to have much luck getting much better than that though.
At the moment; I’m mostly expirementing in the 70cm, 33cm, 23cm and up in the 5cm world. MOSTLY I’m space constrained keeping me out of HF/VHF, But unitl I have a HAM licence to get into TRANSMITTING I’m somewhat less interested in skip and LONG range comms.
I had picked up a RF Explorer and 6GHz Signal Generator a while back (early 2015) as a toss-around Spectrum Analyzer. The Explorer itself is tolerable; from 15-2700MHz the unit features:
- -105dB ADNL
- 10 ppm frequency accuracy + stability
- +/- 1 dBm amplitude stability
- +/- 6 dBm amplitude accuracy (this has burned me a few times)
The unit I have is the 6G Combo unit; which also features a 4.85-6.1GHz front end as well. I intend to extend some of the software included with the unit and building a simple X-Y RF-transparent gimbal for directional antenna testing at some point. (though going to need a LOT of space for that!)
The 6G SigGen unit is by far the more useful unit of the two; featuring 24 to 6000 MHz output range; 0.5 ppm frequency stability from 0-45°C, internal programmable attenuator, and features frequency sweep and CW output. Shame it doesn’t offer any modulation methods; but for the price of the thing, I can’t complain.
The Antenna analyzer I primarily use is an older JDSU/GenComm GC7105A, which is limited from 25MHz - 3GHz. (there’s a 4GHz software upgrade option, but I can’t find a code online, and the company doesn’t sell them anymore.) The built-in Tracking Generator and directional couplers make it a wonderful blend of a network analyzer + spectrum analyzer + power meter, though the maximum 250 points, the slow sweep speed, and the fairly high noise floor (~-100dB depending on span + range) limit the long-term interest I have in keeping the thing around. I Managed to pick this one up used from a Korean test equipment depo back in ~2014 for an insanely low price (as there’s scratches on the screen, and some physical damage on the tracking-generator N-connector, none of which affects operation).
While working on that; and having just put the “test fixture” together; I figured I should test some of the various inexpensive 50Ω loads I’ve picked up; and build a low-quality “50Ω axial resistor dummy load” to test at the same time. I’ve got:
All of the “rated” loads tested just fine in the 25-3000MHz range that I’m testing them in.
For anyone who isn’t farmiliar with why carbon-film resistors with axial legs aren’t used at high frequency; lead inductance is a pain in the ass!
For my own sanity; I built a 0.5W 50Ω terminator using a pair of parallel 100Ω 0.1% carbon film resistors to verify the inductance vs frequency does in fact lead to a climb in reflection.
Built the test circuit in a hammond metal test box; and threw it in the fixture and confirmed that my understanding was true. (only swept from 140-550MHz; as I was too lazy to recalibrate the analyzse)
The HobbyKing (rebrand) X120 Battery charger is just terrible. To the point I might just need to crack the thing open and see what I can do about modifying it.
When disipating a load; the display is reading about 55% of the actual total consumption value (assuming the HobbyKing power meter is correct) I’ll have to pass this through an external meter later to see which one is lying. (The power meter I feel like I’ve checked in the past and found it to be within 1%) I’ve always taken the displayed “charging mAh” to be correct; as it generally matches my expected values (if I draw 80% out of a 1.3AH lipo, I expect to see ~1300mAh * 0.80 => ROUGHLY 1AH go back in; and it’s usually pretty close) I suspect the load sense resistor is likely the wrong value; or damaged. (or simply too low that the sense circuit is burying the value in noise.)
Further to the above topic on Batteries; the “load” portion of the HobbyKing X120 is just useless for large batteries. It takes over 6.6 hours to discharge a 4AH 3S battery; the load being limited to a measly 10 watts. (two orders of magnitude lower than just putting it into an RC copter and FLYING the thing)
I’m really hesitant to simply BUY an electronic load; they’re SUPER expensive and I always hate test equipment software. (though as long as the thing provides data over USB/LAN; that can always be fixed!) I’m tempted to try building one of the Arduino Dummy Load projects; but may go for a somewhat larger (~300 - 500W) range for battery discharge testing.
I’m also somewhat tempted to look at acquiring or building a four quadrant source-measure-unit; Though that get’s even more expensive fast!
All that; and now on with the evening! Might play some computer games; might continue working on the electronic load; might take a look at Brynn’s bike to see what it needs to be temporarily rideable. (She’s convinced she doesn’t want a new one “yet”)