Thursday, February 24, 2011

Need Coax? Contact ABR INDUSTRIES

Looking around Houston for somewhere to buy some coax that wasn't the no-longer-accurately-named Radio Shack I discovered ABR Industries (American Built and Reliable). Perhaps you've seen their advertisement in QST but not read any further and discovered they're located right in in Houston.

I've just purchased a 50ft RG8/X cable with silver plated PL-259 connectors and weather proofing for $32.95. I was impressed by the quality of the components used, the build and care taken in assembly.

Check out the website and a range of coax & accessories. If they don't have it advertised they also do custom cables, connectors & lengths.

The owner of ABR Industries is Marc Abramson, KC9VW

Coax cables and shielding.

After looking into the minimal amount of shielding on my bargain-bin coax I decided to do a little research on the difference shielding can make to signals traveling through coax cable.

The following information is from
 So, what's the result of these differences in shielding? Engineers measure the effectiveness of a shield by what is termed "transfer impedance," which indicates the extent to which a signal outside of the cable reaches the inside. Transfer impedance is a function of frequency, so to gauge the relative effectiveness of shields, one has to know what the frequency band in question is, but at all frequencies, a precision video cable with a 95% braid and foil outperforms quad shield cable. The following table is taken (with permission of the author) from the Audio/Video Cable Installer's Pocket Guide, McGraw-Hill 2002, by Stephen Lampen, an engineer with Belden Wire and Cable. It shows the transfer impedance for various shield configurations on RG-6 type cables at various frequencies; the lower the number, the better.

My bargain-bin coax would be lucky to have 60% coverage and there was NO foil. Clearly a case of "you get what you pay for" and let the buyer beware. The foam dielectric was very soft ... it could be permanently deformed with just my fingers. I shouldn't have been surprised to have problems with high SWR after running that coax under carpet and across walkways!

Wednesday, February 23, 2011

Notes on a 20-15M small transmitting loop

From slogging through the loop calculations and formulas I think it would be possible to build a loop to cover 15M, 17M & 20M. It would be 3.3 Ft in diameter ... more than a foot smaller across than my 40M double loop. The efficiency looked like it was going to drop a fair bit from 15M to 20M but then I realized that loss would represent much less than 1 S unit. The calculations were based on a 11 Ft circumference of 3/4 in copper pipe.

Frequency Capacitance Required Ham Band Efficiency
21 Mhz 24 pF 15M 89% - 0.5dB below 100%
18 Mhz 33 pF 17M 83% - 0.8dB below 100%
14 Mhz 55 pF 20M 66% - 1.8dB below 100%

In theory it would be possible to operate the loop down at 40M with a large enough capacitance but with greatly reduced efficiency ... perhaps a few S units down (8.3 dB below 100%) ... you could push 50W and still fair ok with PSK31 and Olivia in a pinch.

When is a clothes-peg not a clothes-peg?

When its a gamma-match spacer for a loop antenna!
The gamma match of my antenna is a piece of large gauge copper wire, soldered to the copper pipe of the antenna body on one side and secured to a plastic bracket on the other. I needed it to move out just a fraction more than it wanted to optimize the impedance match between it and the transceiver. To keep things in place a quick search around the house brought forth two plastic clothes-pegs a little different to the one pictured to the left.
Initially I wondered what affect the metal clips inside the peg might have but it seems they make no measurable difference.
Bottom line is: Don't be afraid to experiment and challenge the notion that "store bought" is better. You might be pleasantly surprised by what you can create and how well it performs.

Thermal drift and increased SWR - Part III

Thanks to WB5TKI who kindly loaned me a spare 50ft of coax, I am now fairly sure that my bargain-bin coax was responsible for the rising SWR I had experienced the last few days.
I wasn't able to run a really long session last night but the few QSOs I managed were satisfying in a few ways:
  • The SWR seemed steady while transmitting. 
  • When tuning the antenna using an analyzer at the rig end of the coax I'm able to realize very close to 1:1 SWR.
  • I made my first DX contact of a significant distance and crossed the North Atlantic Ocean when I responded to a CQ from F5TTI in Vermelles, France. F5TTI has some fairly impressive antennas so I think that had a lot to do with pulling my signal out of the aether ... I'm still calling it a win though :-)
I've enjoyed the 40M loop antenna so much I'm working up plans for a 20M-17M-15M loop to compliment it. N5IDF has expressed interest in building one as well so there may be a few more of these unique antennas being used in the future.

Monday, February 21, 2011

RCA WR-99A Crystal-Calibrated Marker Generator

Well, what the heck is it?

Its a fancy radio frequency signal generator made by RCA (Radio Corporation of America) and used to service television and radios.

Original cost was approximately $260 and coverage was from 19 Mhz to 260 Mhz ( ... or 19 Megacycles to 260 Megacycles)

The frequency range is above normal broadcast bands so it is not really designed to service AM radios ... mostly FM radios and television sets.

A precise (hopefully) crystal oscillator provides markers every 1 Mhz or 10Mhz to aid in setting the variable frequency (it has an inbuilt heterodyne detector and audio output) or you can use a frequency counter like I have to accurately set the output. There is also a 4.5Mhz marker for IF alignment.

The switches probably need a shot of contact cleaner but otherwise it works just fine. Its not a piece of equipment that would be used very much nowdays but its an impressive beasty when it lights up and the band change controls change over with a satisfying thump.

Oh ... yes, it does have tubes that light up inside ... so qualifies as "real" radio equipment :-)

RCA WR-99A Crystal-Calibrated Marker Generator

Sunday, February 20, 2011

Thermal drift and increased SWR - Part II

After some not-very-scientific testing I've determined that there isn't enough heating in the antenna to affect the tuning to any noticeable degree. The next possible culprit at this stage seems to be the coax.
Now that I have had time to think about it the coax has been abused enough lately that it is entirely possible it was pinched ... it is cheap enough that the insulation is a foam dielectric and one pinch would move the center conductor unacceptably close to the little shielding it has. Prolonged PSK31 transmissions would heat the pinched area ... further compromising the insulation and increasing the reflected power ratio.
Something else that cropped up during testing was the different resonant points for the loop antenna when measured near the antenna (but not near enough to affect tuning) compared to the end of the coax ... 7.035Mhz at the antenna & 7.040Mhz at the rig ... doesn't seem right at all.
Time to get the wallet out and buy some better coax ... this time I'll run a length of through the wall to the loop antenna, out of the high traffic areas!

Wednesday, February 16, 2011

Thermal drift and increased SWR

Along with just about everyone else on the planet we've been experiencing seasonably "different" weather for the past few weeks. Texas has been particularly cold, with a lot more freezing than we're used to! This cold snap happened to coincide with the first real heavy use of my Kenwood TS-140S & small transmitting loop antenna.
Well, now that the weather has warmed up, I've discovered why the original builder of my antenna had clamped the capacitive tuning assembly securely together. It seems that while continuously transmitting, the energy running through the antenna is enough to cause a very small amount of heating, this heating is just enough to shift the VERY sensitive tuning elements and de-tune the antenna slightly!
I had left the clamps loose while I moved the tuning of the antenna and since it is located in a low traffic area (the garage) I didn't see a need to tighten them again. The freezing temperatures must have kept any heating to a minimum or at least below a critical value and the tuning remained solid.
The whole antenna is made from copper pipe and has fair coefficient of thermal expansion so it shouldn't come as a surprise that a change in temperature will cause the tuning elements to shift. I'll be interested to see how stable it can remain when I clamp the elements back together again.

More experimenting required!

Worked All States - Incentive for improvement

Ever since I saw the Worked All States (WAS) award on the ARRL web site I thought it sounded like a challenge I could get into. Its not a competition against anyone else so I don't feel like I'm competing against "Big Gun" stations with thousands to spend on high end rigs or huge antenna farms. Not to mention the home owners associations or the CC&Rs which forbid a noticeable external antenna in our neck of the woods anyway.

Simply put, the WAS award is given to a radio amateur who can prove a two way radio contact with all 50 US states using any band or mode he/she is authorized to use. The lower frequency bands such as 20/40/80 Meters are suitable for reaching out across the US so my loop antenna (All 4.5 feet diameter of it) will work as it resonates on 40M and is currently tuned to 7.035Mhz

How many people have I talked to? Well I haven't actually "talked" to anyone as there isn't even a microphone plugged into my rig at the moment. I use PSK31 and other digital modes to contact fellow amateur radio operators. My computer encodes what I type as a series of audio tones and then transmits those instead of voice. On the other end a radio receives my transmissions, demodulates the tones and then passes them onto the computer which decodes them back into text.

Twenty or thirty simultaneous conversations (QSOs) can occur on one frequency so it makes very efficient use of the available bandwidth. Also, very little power is required. Typically 25 Watts is all that is required to reach from coast to coast from here in Texas.

Below is a map of confirmed contacts that will count toward my WAS award. I've made contacts as far away an regions of Canada, Mexico, Cuba & Guatemala but they don't count in this particular case. To be a confirmed contact both parties must submit log information to a third party which doesn't always happen!

Loading the same log book information with 67 logged contacts into both and LoTW results in 31 QSL records for eQSL and 13 QSL records for LoTW. I think the ease of automatically uploading logs into eQSL has substantially aided its adoption.