Updated 10/19/11

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This is an extra page to store some of my "stuff."

wired op-amp

Current Feedback OP-AMP as Broadband 10 MHz Amplifier For Radio Astronomy Project.(updated 10/19/2011)

The amateur radio astronomy project is being sent by email as each part is completed. The sum will be presented at Ozarkcon 2012 in Branson MO. Parts I through III have been sent out. Email me if you wish to be added to the email list. My email is aldenmcduffieATsunflower.com. Remember to replace the AT with an @.

The experimental telescope will require a wideband amplifier to collect the noise signal obtained from the antenna. Our goal is to detect our sun and then Sirius. I've been experimenting with the TI current feedback opamp as a candidate. The picture at right displays my copy of the THS3001 evaluation board for that opamp.
wired op-amp

It meets the needs so far. I drove it with 0.7, 7, 70, 700 and 7000 KHz sinewaves, all with a 0.1 VPP input. As expected the response was flat all the way out to 7 MHz! Remarkable. At received the amp has a gain of 2 in the inverting input. We'll be experimenting with increasing the gain as much as possible, likely splitting the total gain needed, over 30, into two amplifiers.The scope trace at right shows the signal wave at 7 MHz.


New FT450D XCVR, all HF bands.(updated 10/1/2011)

FT450D is at lower left. LDG-450 tuner immediately above on shelf. Astron SS-30M at right. I hear no switching interference. HDTV upstairs sees no interference, ala XYL! DAIWA switch above routes XCVR output to 10-WATT dummy load - so can practice with Bencher Keyer - or to antenna. Can also use QRP 5 Watt Step Attenuator for transmissions down to 2.5 mW (not shown). Fun!

scope trace

Shinny new K1-4 Kit arrived 9/1; now on the air 9/18 and loving it: 40-30-20-15 M QRP, WØXI
(updated 9/18/2010)

I really liked the kit, as many know well documented and goes together pretty easily. I made one assembly error, leaving a blob of solder across the VFO leads at the PCB connector: result, VFO running but no tuning. Blob removed and all was AOK. I found my IC-703 (TX burnt out) to still be useful in using it to check the K1 transmit frequencies. It's now my backup FREQ Counter, hi.

First contacts 9/18 were a couple of answers to CQs for those running the Salmon Run QSO party in Washington state! Got a 55N from N7AAL. Wheee! Third contact was with N1LGW in the northeast, nice but quick QSO. Way cool; what fun!

scope trace

Building a Short Helical Antenna for Shortwave Listening and
40, 30, or 20 QRP
By Phil Anderson, WØXI
(updated 9/18/2010)

If you have limited space for a shortwave and/or QRP antenna – and perhaps nosy neighbors too, the short helical just might be your ticket.  Simple wire antennas, such as the end-fed vertical or dipole, work well for listening from the AM band through 10 MHz. The problem is, of course, that they work best when the antenna elements are roughly one quarter-wavelength for the frequency of interest. For example, a quarter-wave end-fed vertical for listening to WWV at 5 MHz would be best if cut to 46.8 feet! You might as well put a bright red blinking light at the top to attract the neighbors and bats! A dipole can be strung in the trees much lower; but, your trees or structures then need to be 100 feet apart or thereabouts. These structures require the use of a ladder in most cases too, not my choice! So, let’s return to the humble helical.  The helical can be as short as eight to ten feet and does just fine. All your work will be from the ground but you’ll have to wind the big coil; life has tradeoffs!

The current state of my helical is shown in the picture at right. It's ten feet of PVC pipe with a small cross-hairs aluminum capacitance cap and seven turns of #18 AWG just under the cap at 4.25 inches per turn. The remainer is a straight drop of #18 to the bottom of the PVC. It’s held at the bottom by a plastic potty ring, compliments of the local ACE Hardware store and at the top by three nylon “guy” strings running to the back and east fences and to the porch steps. I added a little box at the bottom to house an SO-239 coaxial connector, the center pin of which connects to the bottom of the coil. While not apparent from the picture, I drove a five foot copper ground rod into the earth right next to the base of the helical and ran a wire from that to the ground side of the SO-239. Also not visible are three ground radials of 33 feet each. Using my shinny new Autek Research VA-1 vector analyzer, I found the antenna to have a minium SWR of 1.58 at 14.02 MHz with exactly 7 turns for the under-the-top coil as noted above. An SWR of 1.7 was measured at 13.7 and 14.3! Not bad.

If the 4.25 inch/turn coil is extend all the way to the bottom of the PVC, then one has a resonant vertical at 10.1 MHz with a VSWR measured to be 1.72. I may try adding some relays and this coil so can switch operate to either 30 and 20 meters.

If the full coil is wound with 2 inches/turn, resonance is just below the 40 meter band. I used that configuration last month to do some short wave listening with a crystal set with success. I didn't stop to adjust/meaure the reduction in turns necessary to resonate it at 7.060, i.e. QRP for 40-meters. Obviously, that's doable. The general rules of thumb when fully winding a helical are to use a half-wavelength of wire at the center of the desired frequency of reception and to space the turns so that the length of the helical is somewhere between 5 and 10% of a full wavelength of the center frequency of interest. With 52 turns spaced 2 inches apart, the coil length for my helix came out to be 8.6 feet, or 5.8% of a full wavelength. That’s a short SW antenna!

winding helical antenna

winding the coil

The picture at right shows how I wound the oringal SW coil on the 4.25 PVC form. I drew a line the length of the ten-foot pipe and marked off ticks every two inches. I then laid the form across two tables as shown and wound the coil off a spool of #18 wire, taping the wire with duck tape every six turns (one foot). Once I settle on the final length, I’ll spray the assembly with a lacquer coating of some sort to keep the turns in place.

When installing the helical for portable use – say in a park – it’s easy to secure the potty base and guy wires with some plastic tent stakes, again courtesy of your neighborhood ACE Hardware store. I got the nylon twine for the guys there too. Radio Shack still carries lengths of 50 ohm coax, ideal for use with this antenna. For use with a crystal set, you’ll need to inductively couple the far end of the coax to the coil of the set (or tap the center tip low on the full version coil).

Performance of  verticals and the “normal” mode of a helical vertical

A quick modeling of an ideal yet simple 6.0 MHz quarter-wave vertical above excellent ground was made using the computer program EZNEC (ref 2). Graphics of the standing wave ratio (SWR) and vertical radiation pattern are shown at right. Note, typical of verticals, that the maximum radiation is roughly 30 degrees above the horizon (ground) and the same in all directions. This is ideal for listening to stations some 500+ miles away. Very little energy is received from sources straight up! The antenna impedance at the feed point (bottom of the helical and ground) is typically 33 ohms. By using 8 wire segments forming two helical turns as an antenna, I got a sense of how winding the antenna as a helical affects the radiation pattern and impedance.  I used dimensions similar to those in the actual helical above. The radiation pattern became a bit more rounded, able to receive a bit more energy from straight up and the bandwidth narrowed a bit.

After winding 52 turns on my (original) vertical and installing it with an imperfect ground rod with three added radials, the resistance of the antenna  – when cut to resonance – increased to 71 ohms and 0 ohms reactance. The added resistance above the theoretical best of 33 ohms is no doubt due to earth and antenna ground losses. I plan to add a number of radials, which should pull this radiation resistance value down to less than 50. For reception use, this slightly high resistance is not a problem. If the antenna is retuned to 7.0, 10.1 or 14.06 MHz for amateur radio transmissions as well, then the added ground rods and a match to the 50 ohm coax will improve performance a bit.


  1. A helix acts like a simple vertical when its diameter is very small compared to its length. This is called its “normal” mode of operation. When its length and diameter are roughly the same, it acts like a coil and radiation of predominately in the axial direction, kind of like a magnet.  (Ref 3.)
  2. Dictionary definition of a helix: something spiral in form, a coil formed by winding wire around a uniform tube.


  1. RX Vector Analyzer, Model VA-1, an antenna analyzer, purchased from Autek Research, www.autekresearch.com
  2. EZNEC, An Antenna Simulator Program, by Roy Lewallen, W7EL; see www.eznec.com
  3. Kraus, Antennas, p 175, McGraw-Hill, 1950 (discussion of normal and axial modes of the helix).

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