Mobile Stereo NATURAL RADIO, part 2

If you've looked at my schematics there's a couple of things common to both the vertical E-field and H-field loop amps. Namely the arrangement of antenna input protection, high impedance matching, high gain amplification, LC low pass filtering , remote gain control and balanced output for signal and power supply.

A lot of experimentation went into developing these amplifiers even though little of it is original work. Some was chosen because it's obviously appropriate and basically sound design. Some is modified to improve the original design and overall the components are assembled to provide better and more flexible performance than any of the original designs offered. All of the components are available at this time from Mouser Electronics as well as others. The electronic volume control chip is offered by JDR Microdevices at a good price. Including remote gain control solved a lot of problems in system performance especially when you consider erecting a heavy antenna array. Taking it down several times to get levels correct for best signal to noise ratio is tedious at best. Putting up the array and setting level remotely from the comfort of a vehicle makes a lot more sense. The chip used handles this task with ease and requires only a remotely located 50K potentiometer. Under differing signal conditions this feature becomes very handy.

The choice of JFET for the vertical amplifier was arrived at by testing virtually all of the available transistors. It has high impedance input and gain, is configured for good filtering characteristics for RF signals above 10khz and gate protection via the back to back zeners. The LC low pass filtering is almost exactly taken from the BBB4 receiver by Steve McGreevy.


Using the MC3340P chip for remote gain adds a -85db to +6db control range and a lot of convenience. The output stage, although very simple, allows the signal obtained to be sent down cable in balanced fashion with good common mode rejection of hum and noise and an easy way to feed operating power to the amplifier.

 The U310 is available from RF parts company and they have a $25 minimum order policy. Other devices will certainly work. MPF102, 2N3819 and 2N5486 are good choices just note the different pinouts for each transistor.

An antenna and mount for this type of amplifier is very simple and many choices are available. The choice of loop amplifier can take several forms. The modified transistor version based on Renato Romero's loop amp as modified with LC low pass filtering and remote gain control is very good and uses commonly available transistors.

H-FIELD ANT. AMP (V.4)  Based on R. ROMERO's amplifier modified by D. EWER for remote gain control and 600 Ohm balanced feed. 3-22-2000
Q1 = 2N5088 or 2N3904 
Q2 = 2N3904
Q3,4 = MPF102
Q5 = 2N3904
T1 = Mouser 42TM117, 1k-50k Transformer loop step-up Z
T2 = Mouser 42TU011, 42TM011 or 42TM022 transformer 1.5k-600 signal out balanced and +12v in on centertap
IC1 = MC3340 electronic gain control IC -85db to +6dB range. 50k to gnd remote pot.

 The AD620 version using the step-up transformer input matching and zener protection has significantly more gain and slightly better noise performance along with the filtering, gain control and balanced output. JDR Microdevices can provide this instrumentation amplifier in single lot quantity for around $8 each. The choice of antenna configuration and size is about endless.


Shielded loop antenna amp. features balanced input for loop and 500 Ohm balanced output. AD620 amp. has gain setting trimmer, back to back zener diode protection, PI LC low pass filter and remote gain control stage using MC3340 with -85dB to +6dB range. 4 conductor cable provides balanced output to natrad mobile MK4 stereo box and +12VDC and ground plus remote gain control with 50k pot in MK4 box. D.E. 4-4-2000
Will Payne's Octoloop is one of the best I've used and is very easy to construct. Information on how to build one is available at . Matching the loop to the AD620 is best performed using a step-up transformer in balanced fashion on input and output. Operating the amp on a single 12 volt mobile supply is accomplished easily by biasing the centertap to VCC/2 and the output can also be connected to this voltage with pin 5 or it can be connected to ground for a zero output voltage. 

Some suppression of rectified RF voltage is obtained by connecting 150 pf capacitors from both inputs to the gain control pins. Much more information on handling the AD620 is available at .

Bringing the signals these amplifiers provide to your vehicle is accomplished with 4 conductor cable. The choice of cable and connectors is quite broad. My version uses surplus cable arranged in two pairs of insulated and shielded wires with an additional bare ground wire. All of the conductors are 22 gauge and the cable has a PVC jacket. The plugs and jacks are from Radio Shack and are 4 conductor Microphone units used for CB radios. The choice of box to house the balanced transformers and power feed system can take about any form. Mouser supplied the cast aluminum box and transformers I chose. You can also combine a tape recorder supply for a mono or stereo cassette recorder or what have you into this box for convenience sake. The output of the balanced transformer can be isolated from common supply ground to feed cassette recorders. The Radio Shack Optimus and similar units have signal ground and supply ground conflicts so keeping the output winding isolated from ground is essential. The Marantz stereo PMD430 has none of these ground conflicts so even with a positive supply ground the audio inputs can be connected to the vehicle negative supply ground. I chose to isolate both the microphone and line level outputs to keep the equipment more flexible. Note the work is pretty much done by the antenna amplifiers. The schematics, layouts and PCB designs are all available or you may chose to design your own. The equipment as shown here works very well and the portable antenna mast and the use of both the loop and vertical antennas for stereo reception really provide a lot more interesting approach to natural radio reception and recording. More work is currently underway on methods to get rid of 60 hertz interference and many of the offending harmonics. Check back from time to time for those results and by all means offer your own ideas or improvements to these pages. You can reach me at my email if you want more information or just want to chat about natural radio. See at:

Mini MK4 Stereo Box
Mini MK4 Stereo Box
Uses 4 conductor flat phone line and RJ11 plugs and jacks between antenna amps and box. Provides +12VDC and ground return plus 600 Ohm balanced signal input and DC isolated line and MIC level outputs for a stereo or mono recorders.
All gain and filtering provided by antenna amps.
By D. Ewer 4-4-2000

MK4 Stereo Box
MK4 Stereo Box
To run 2 antennas and remote gain controlled amps. New antenna amps have all gain and filters. This box provides +12VDC and groud return plus the remote gain control and 600 Ohm balanced signal. The record outputs include line level 5 kOhm and mic level signals that are DC isolated from the vehicle system and the antenna amp.
By D. Ewer 4-5-2000

MK4 Stereo Box and MK5 Stereo Monitor
MK4 Stereo Box
for use with E and H field antennas and amps. Box has remote gain controlks for antenna amps and DC isolated line and MIC level outputs. Power is supplied by 12V vehicle battery and each chan. is fused and switched. A positive ground 4.5 or 6.0V recorder supply is also provided. An 8PIN DIN jack send: power and signal to a MK5 stereo monitor. Radio shack CB MIC plugs and jacks are used for power and balanced feed to antenna amps. D. Ewer 3-22-2000

MK5 Stereo Monitor
Numbered parts from MOUSER. Volume controls with switch from radio shack. Excellent for multiple listeners and when a stereo recorder is not available. LM386-4 has superior performance to previos chips. D. Ewer 3-8-2000
Click here to view the scheme

MK4 inside boards MK4 front view
MK4 Front panel back MK5 Stereo Monitor

Resources: Renato Romero, Dave Oxnard at the workshop, ARRL, Analog Devices, Motorola, The Longwave Pages and Will Payne, Steve McGreevy, Mouser, JDR Microdevices, Radio Shack, RF Parts Co. and others.

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