Do you want to measure antenna impedance at resonance? With this Antenna Scope, you have a simple RF Bridge for getting started in an exciting part of Ham Radio -- Building your own Antennas that work well! Additional information on the pixie2 is available elsewhere on my Web Page. Just follow the Pixie2 link. -Ed Loranger, Amateur Radio Callsign WE6W http://www.qsl.net/we6w --------------------------------------------------------------------- I built this thing many years ago, and used it recently to build my Folded Bazooka Antenna. OK. I don't want to hear about "How inexpensive" this or that MFJ or whatever antenna analyzer is. I don't have the luxury of ANY spare cash. So I build everything unless I get a trade from someone. You could build a Chopped and Amplified noisesource to drive an RF bridge, but that project is a bit more involved and you might not get to it. Presented here is something you can build quickly and really is something you can build TODAY! And it works. It is driven with a GDO or other signal source at the design frequency and the input is inductively coupled. Do not connect your signal source directly as that will load down the bridge -- destroying your measurements. (First hand experience on that mistake :) Basically, the GDO signal drives two legs of a bridge circuit. Any difference in the two legs is Detected by D1 which will charge throught (M) and (C2) will hold that charge. Your goal is to set R1 to 50 or 72 Ohms (Ant. Impedance to cut your antenna to), and trim the antenna until (M) measures NO Current. I.E, balanced condition. The circuit works with both balanced and unbalanced antennas. Here is my ASCII dwg, and you can get more information from "Radio Handbook" William I. Orr, W6SAI. USE FIXED WIDTH FONT +----------+------------+ | | (C1) ) (R1) | L1( | (R2) ) +---|>|------+ ( | D1 (R2) | 0 | | Ant/Load +---(R3)---+-----+ | (C1) | | | | (M) (C3) +----------0------------+----------+-----+ L1: 4-7 turns input loop for injecting signal from GDO. R1: 100 Ohm, non-inductive Potentiometer. Ant/Load: Connect antenna or Resistive loads to calibrate. C1: Two of these, 500 pF close tolerance Silver Mica capacitors. R2: Two of these, 200 Ohm Close tolerence (Measure to match.) D1: Germanium signal diode (1N34A) R3: 1Kohm resistor C3: .001 disc ceramic capacitor. M : 0-200 microamp meter or Zero center. ------------------------------------------------ Calibrate a scale for R1 using known resistive loads across the Ant/Load jacks. Inject a signal by coupling to L1. Adjust signal input to get 3/4 scale reading on the meter with no load connected. Connect 50 Ohm load at Ant/Load terminals and verify the bridge balances when R1 set to 50 Ohms. Connect Antenna where 50 Ohm load was. Trim antenna to desired Impedance. NOTE: Connect directly to Antenna Input, not to your transmission line, unless your transmission line is absolutely KNOWN to be an electrical 1/2 wavelength long. I hope this is a useful contribution. -Ed Loranger, WE6W (Posted to QRP-l on Feb. 20, 1998) (c) Ed Loranger. --------------------------------------------------------------------- Feb. 23, 1998: Folks, I used my own posting last Friday to build the Antenna Scope. It works FB! What a great use for a GDO. Not only does it Null very nicely and give you the impedance of the resonant antenna, but I connected the output of my pixie2 to it and measured 140 Ohms output impedance, so I added another 220 pF cap across the first PI filter cap connected to the transistor. Now my Christmas Pixie2 is up to nearly 800 milliwatts output! One note, I couldn't find a 100 ohm or 250 ohm potentiometer for part of the bridge section. BUT! I found a 10 Turn, Non-inductive, 1 Kohm pot. Linearity +/- .2%!! It is 100 Ohms per turn and I only had to mark the 0/100 Ohm, and the 50 Ohm location on the scale. Nice and easy to interpolate the antenna resistance. So as a test, I connected up my old 7040 Dipole and it was resonant at 6600 KHz. Now trimmed to 7040. (This original dipole was never measured, just 468/F(Mhz) feet. It was too long because of the Velocity Factor of the coated wire. Another 7040 Dipole I cut using a SWR meter was cut too short! It was resonant at 7180 KHz. I'll have to adjust that puppy. Anyway, the GDO is a nice input signal but you can use a QRP rig and inductively couple a weak signal into the Antenna Scope and start cutting antennas well. Of course, a noise bridge is probably easier and better to use, but my antennaScope only took 1 Hour and 15 Minutes to build from junkbox parts. No battery needed. -Ed Additional Information: A GDO is a "Grid Dip Oscillator". Basically an oscillator with a broad frequency range. My GDO it tunable from 400 KHz all the way up to 200 MHz. A great continuously variable RF source. You can measure Capacitors, Inductors, Antennas, and Tuned circuits. The meter on it registers a DIP in plate current (Like the FINALS on a TUBE transmitter) when in the presence of a tuned circuit. A Pixie2 inductively coupled to the AntennaScope can also serve as the RF Source. You can also couple the output of another Low power transmitter and drive the unit with that as well. (Inductive loop coupling.) This low power is under 100 milliwatts or you my overdrive the antennascope. On the Pixie2: I have mine heatsinked and am using a 2N5109 for the final. It doesn't put out any more power than the 2n2222A, but the 2n2222A doesn't live very long -- as you mentioned. Actually, my 2n2222A put out 800 mW, but got very hot. So I put in the 2n5109 for safety. Measurements made on the Pixie2 are considered 'Course', since the unit is checked while un-powered. Results may vary.