Thursday, November 27, 2014

PL-259 and SO-238 Characteristic Impedance





So let's run the numbers on a PL-259 connector and SO-238 socket. Unfortunately they omitted the "D" dimension on the above drawing but we can guesstimate a little.

Characteristic impedance Zo = 138 Log (12/5.5) = approximately 47Ω. Granted, if it is a little bigger say 6.0mm then it's is approximately 42Ω.

Back when I was a repair technician for a Motorola Service Shop, Motorola produced an 800 MHz radio that had an SO-238 antenna connector! I couldn't believe it but it worked and now I know why.

Myron WVØH

Wednesday, November 26, 2014

RCA (Phono) Connector Characteristic Impedance

I was overhearing a conversation about RCA connectors and their characteristic impedance. I decided to "run the numbers" on a typical RCA connector.


Characteristic impedance Zo = 138 Log (8/3.2) yields approximately 55 Ω.

Keep in mind that the phase length of the connector is so short on HF that the characteristic impedance hardly matters. While it does provide an impedance bump in the time domain, the impact to an HF system is negligible. On VHF+, it may come into play depending upon the application but even so, a 5 Ω variance is low enough that it may not cause an appreciable change in signal integrity.

Myron WVØH

Sunday, November 9, 2014

North Georgia QRP Club - Peanut Sprint Results

The results are in or out for the Peanut Power Sprint.

The contest is a QRP field contest placing an emphasis on, of course, field ops but not excluding those with home stations or high power. So I decided to partake in this test because of the publicity on the QRP-L email reflector. So I thought, "what the heck, I have nothing to loose."

I went out to the local park to operate and found that my ops were casual at best and there were many dry spells in QSOs but stuck with it and found myself having a great time as well.

My Park Portable Doublet was in order as I decided to try the Goober category as a 1-Watt maximum power output entry. This placed an extra challenge aspect as QRP is hard enough let alone the 1-Watt level.

My rig was the Elecraft KX-3 and the BLT I made from scratch.

All in all, I had a great time and didn't do too badly.

73 all, 
WVØH

Monday, October 27, 2014

Magnetic Loop RF Safety

Since the RF E field density is reduced in strength by a square of the distance away and magnetic field strength reduces by a cube of the distance the magnetic field strength at a given distance is generally weaker than the E field of a wire antenna, except with a magnetic loop. The magnetic field intensity of a mag loop is considerably higher than the E field probably due to the inductive portion of the resonant circuit creating the radiation (magnetic) in the near field.

I ran the numbers on my modified AA5TB loop calculator spreadsheet using the following assumptions:

14.1 MHz, 2 foot diameter loop made with 0.5" copper conductor and 5 Watts of RF at a distance of 1 foot away, a small loop advertised as being usable as a manpack antenna close to the human head.

Results at 5W = 3.8A/m and at 2.5W = 2.7A/m. A non-compliant condition. 

The FCC max is 0.347A/m for a controlled environment, however the 6 minute time averaging has not been factored into this, this applies to 100% keydown for 6 minutes minimum. 

So we would have to factor in duty cycle and scale back the input power accordingly. But just wanted to shed some light on the distinction between using a calculation for E fields when the H field should be evaluated.


Friday, October 24, 2014

MFJ-2289 Big Ear Balun Evaluation

I heard someone say (read on eHam) that the balun inside the V-Block of the MFJ-2289 Big Ear was trash and they replaced it with a "better" balun.



Hmm. That's curious because I had inspected the 1:1 Guanella current balun when my Big Ear first arrived because I just had to take it apart and see what was inside, and didn't see anything alarming about the design or implementation.


So I decided to do what any curious person would do, test it and verify the loss myself.

I took another 1:1 Guanella current balun that I built and attached the two back to back after unsoldering the wires from the 3/8 x 24 studs inside the box.


I calibrated my test bed by attaching the output of the Kenwood TS-2000X set to 10-Watts with the test cables attached directly to the Elecraft DL1 dummy load and the test point attached to my Fluke 89 IV, I established a reference level of 9.911 Watts in the "thru" path.



Pthru(W) = (15.591 + 0.15)^2 / 25 = 9.911 Watts, through calibration path 
Pdut(W) = (15.163 + 0.15)^2 / 25 = 9.3795 Watts, through DUTs



P(loss) = 10 x LOG (9.3795 / 9.911) = -0.24 dB total for two baluns

Assuming equal power split, where P(loss) / 2 is the loss of just one balun, the measured MFJ-2289 balun loss is -0.12 dB.

Pretty textbook if you ask me. I'd be hard pressed to build a better one.

- Myron WVØH






Sunday, October 19, 2014

Magic Propagation Map

I have a love hate relationship with this map. It leads the average ham into believing that your 10m signal just magically "jumps" over to Europe when nothing could be further from the truth. The thing to keep in mind here is that big old mirror called the Atlantic Ocean is what supports this propagation mode from the eastern US. That first hop landing zone on the ocean is what makes all this possible.


We high plains hams have to endure the landlocked, first-skip-zone-landing-on-dirt propagation of pretty much HF band to get to DX except VE and XE. And it's no secret from any long time DXer, geography matters.

It would be neat to see a map of the actual path that the signal took hopping along its way to grandpa's house. It would reveal that the first hop would put us directly on dirt! In pretty much any direction on pretty much any band from the geographical Midwest, (Kansas, Nebraska). And yes, there are the ducts that occur and showing those patterns and paths would be super cool as well.

Oh, and I love it because it shows when the band is open.



Saturday, October 4, 2014

Last Few Outdoor Ops Stats

I've mined the RBN data to see what my SNR reports averaged for the last 3 field contests, the Colorado QSO Party, New England QRP Afield, and the North Georgia QRP Peanut Sprint all in 2014. 

Along with an average signal to noise ratio I've calculated the standard deviation as well. The std dev could be a result from shifting band conditions. I logged QSOs in two separate locations during the CO QSO Party, in an operating site in the mid-morning hours on 20m with my 40m Park Portable Doublet favoring a N/S path and the mid-afternoon ops favoring an E/W path. In any case, it is interesting to see how the antenna and propagation plays.

The NE QRP AF was operated from 1939-2145Z on Sept. 21,2014 at 1-Watt and The North Georgia Peanut Sprint was operated for the full two hour period from 2000-2200Z on Sept 28, 2014 with 1-Watt as well. The spots were gathered in the Colorado QSO Party on August 30, 2014 from 1635-1730Z at the county line between Jefferson/Boulder county line and 2015-2120Z at Weld county.

CO QSO Party


JEF/BOU
44 spots at 5-Watts
Avg SNR = 13.3dB
Std dev = 7.42dB


WEL
50 spots at 5-Watts
Avg SNR = 9.96dB
Std dev = 8.00dB

NE QRP Afield


100 spots at 1-Watt
Anthem Park, Broomfield, CO
Avg SNR = 8.51dB
Std dev = 5.91dB

NoGa Peanut Sprint


91 spots at 1-Watt
Sienna Park, Broomfield, CO
Avg SNR = 9.75dB
Std dev = 6.66dB