Aero Mobile Ham Radio

Chuck Forsberg WA7KGX caf@omen.com

I built a crystal set from a kit in second or third grade. It had a real galena crystal and cat's whisker. The enamel on the top of the coil was sanded off to allow a slider to select the radio station.

In grade school I made old radios into transmitters by feeding the audio into the grid cap of the 6A8 and coupling the oscillator to an antenna.

In high school I converted a pair of car radios into 6 meter transceivers. I converted a three tube phono amp into an AM transmitter, and converted an FM tuner into an FM transmitter.

In college I converted a war surplus TV transmiter to the 440 mHz ham band. I designed my own modulator with a 7 mHz frequency response. Unfortunately the only thing that picked it up was my TV camera built from a used vidicon and other stuff.

I chose an airplane with a 12 volt electrical system and no ADF wire sense antenna. I wanted to use a wire antenna for HF ham radio.

Aero Mobile with the ICOM 706MKIIg

After some searching I settled on the ICOM 706 radio for use in N2469R. The other candidate was the new Yaesu FT-100. Both radios cover the ham bands from 1.8 to 440 MHz, and have detachable control heads. The 706 seemed the better choice for my purposes. When I compared the two in the store, the Icom had better front end performance in the aviation band. The separation arrangement for the control head was neater in the Icom. The chassis mounted coax antenna connectors on the ICOM are more desireable than the pigtails hanging out of the Yaesu. The traditional availability of frequency mods for Icom radios was a plus. (Frequency mods void the warranty. Out of band transmission may damage the radio.)

The ICOM 706MKIIg covers the ham bands from 160 to 2 meters, plus the 70cm band with SSB, AM, FM, and RTTY modes. General coverage receive is from 30 kHz to 200 mHz. WFM reception is available on the FM broadcast band. The receiver is less subject to cross modulation from strong broadcast signals than a handheld with equivalent frequency coverage. The radio is compact, and the control panel can be detached and remoted from the body of the radio. A IC-7000 with color fishfinder, DTMF microphone, and other amenities is now available for a higher price.

Above: 706 control head temporary installation in N2469R. The MB-63 mounting bracket is attached to a piece of aluminium cut from an old license plate. This is attached to the top of the glare shield with velcro.

Originally I needed an RF ferrite on the OPC-587 separation cable coming from the left of the remote head. With the better grounding afforded by the permament AH-4/antenna installation, this is no longer required.

My initial testing had the ICOM AH-4 antenna tuner connected to a wire antenna (drawn in red) running from the baggage compartment, around the baggage door, towards the left wing, to the tail, and finally towards the right wing. Ropes (drawn in green) attached to the tiedowns and draped over the wings secure the front ends of the antenna. This provides enough length to tune down to 75 meters. The AH-4 hangs on the rear of the baggage compartment with duct tape. That temorary installation could be removed in minutes. I had to be careful to keep the antenna wire from touching the baggage door opening, and to keep the metalized window covers from touching the tuner's antenna terminal.

The carbon microphones of yesterday have been replaced by dynamic or electret microphones and amplifiers. These amplifiers may not be shielded well enough to operate in strong RF fields. The mikes' amplifiers were modified with a .01 on their output and a .0047 on their input to solve EMC problems. Installing the AH-4 tuner directly grounded to the frame, and keeping the antenna lead-in inside the cabin to only a few inches solved most of the EMC problems.

Below: The yellow lines show the permanent installation using standard aircraft components. This is a simple Marconi antenna. The metal frame of the 182 is the counterpoise. The AH-4 is now attached to the shelf immediately behind the baggage compartment. The 706 is mounted on the radio shelf behind the baggage compartment. The ICOM mounting bracket allows easy removal. Only the AH-4 tuner and the antenna are "permanently installed". The FAA Form 337 covers the tuner and antenna.

Ground operation is generally limited to battery power. The drop in voltage reduces transmitter output.

Airborne Ham radio is strictly a VFR affair; it's too much of a distraction while flying IFR. Operating results on HF are similar to land mobile operation. The ionosphere is seventy miles up; a few miles of altitude doesn't make much difference.

One disappointment is the widespread proliferation of strong power line interference. You'd think power companies would be more interested in conserving energy than broadcasting our presence to space alien civilizations.

Left: Mounting bracket for IC-706 is visible at lower left of picture. The radio was removed for the annual. The antenna tuner is above the radio, hidden behind the first bulkhead. The white object is the battery box. The red object is the Energency Location Transmitter (ELT).

I have made a number of contacts on 20 meters, both on the ground and while airborne. I've worked stations from a few hundred miles to the east coast. I also made a 300 mile contact on 75 meters the evening of September 11 on the ground.

Left: Part of the underbelly antenna farm. From left to right: Spare air band comm whip, Comet B-10 144/440 MHz ham antenna, Loran-C antenna, ADF antenna. In flight SWR on 2 meters is adversely affected by installation of wheel pants.

I have made a number of contacts on 146.52 simplex.

VHF repeater operation at high altitudes is a bit tricky as multiple repeaters may be activated.

FM broadcast reception is problematical while flying. The 706 has poor selectivity, image rejection, and front end dynamic range on FM. FM stations on the same frequency are located close together, so co-channel interference is a problem when listening in the air. The 706 could use a lower capture ratio. The 706 audio is thin in the bass, the result of undersized coupling caps.

Medium Wave reception in the air is about the same as on the ground. Unfortunately the 706 does not work well with shorter MW antennas. MW Sensitivity with 10-20 feet of antenna is worse than a pocket radio.

The 706 has two AM bandwidths, one too wide and the other too narrow. A 6 kHz AM filter would be a welcome option. For casual broadcast band listening, the 706 sounds better than the KR-87 ADF, even if the 706 is much less sensitive.

Adding Touch Tone to the ICOM 706MKIIg

Strangely enough, ICOM does not provide a Touch-Tone microphone with the 706. Perhaps ICOM will provide a DTMF mike in the future when competition from Yaesu and Kenwood all-band rigs heats up, and ICOM learns how to make a mike with satisfactory EMC. (EMC = ElectroMagnetic Compatibility, the ability to function properly in strong RF fields.)

In the meantime, I've interfaced a Radio Shack DTMF dialer to the 706. The space previously used by the batteries is used for a DPDT slide switch and interface circuit. The "ON" position selects the tone pad audio and grounds the 706's PTT line.


Three diodes drop the 8 volts provided by the 706 to a voltage more friendly to the dialer. They also provide reverse polarity protection. Most any junkbox silicon diode can be used.

The buffer stage uses a 2n3904 transistor. Most any general purpose NPN silicon should do. The stage is provided to isolate the dialer from voltage spikes. DTMF output level can be adjusted by changing the 100k base resistor. Adjust the gain so the DTMF tones are at the correct level when the 706 mike gain is correct for the microphone in use. Collector current is provided by the 706's mike input circuit.

The 470 and 82 ohm resistors reduce the airplane mike (and DTMF) audio level to something comfortable to the 706. (If the 706 mike gain has to be less than 4 the mike input may be overloading.) The 50 mfd cap in series with the 82 ohm resistor allows the microphone bias voltage to pass. This attenuation works with fixed wing aviation microphones (Lightspeed 20k, Telex amplified dynamic mike).

The apparent overdesign of this interface was prompted by a desire not to fry a second tone dialer module.

I found it necessary to ground the shielded cables carrying mike audio and PTT at the 706, using a terminal strip secured directly to the chassis. Just using the ground on the RJ-45 connectors lets enough RF into the 706 to make it unstable when transmitting on HF. You have to bypass these leads youself, doing the job ICOM's engineers should have done.

My Skylane has a Sigtronics 400 intercom. Even when the intercom is switched off, some diode drops are introduced in the push to talk (PTT) line. The pilot's yoke PTT button only lowered the PTT line to 1.3 volts, not low enough to trigger the 706. The two transistors and two resistors at the upper left of the schematic decrease the "on" voltage to a level the IC-706 recognizes.

Audio from the 706 can be heard on one of three ways. At rest with the engine stopped, the 706 speaker can be heard. Alternatively, a short adapter cable can be run from the headphone output on the remote head to the pilot's phones.

The 706 audio is normally connected to the KMA-24 TEL input. Pressing the TEL button allows the 706 audio to be heard while monitoring other communications. I find I can listen to the 706 at a lower volume than the normal airplane comms, so no muting circuit is necessary.

The TEL output from the KMA-24 is fed into the circuit shown above. Switching the KMA-24 to TEL position connects the headset mike and PTT to the circuit drawn above. This works even when the avionics master switch is off.

In the original temporary lashup, an ICOM battery cable modified with a fuse adjacent to the positive battery terminal connected directly to the battery. This temporary installation could be removed in minutes.

The ICOM battery cable now connects to the master bus at the relay adjacent to the battery. This preserves the master switch's ability to shut everything off, including the 4-10 ma drawn by the 706 when powered down. The 706 is connected ahead of the airplane's ammeter, so current drawn by the 706 does not register on the ammeter.
Point to point wiring inside a Radio Shack tone dialer's battery compartment. The tiny slide switch is the centerpiece of this project. The three dropping diodes are on the right. The buffer transistor is below the switch, the level converter transistors above it.

What I'd like to see in the IC-7000

A few more refinements are needed to optimize the successor to the 706 for use in boats and aircraft.
  • Better EMC protection. This is a transmitter, is it not? Boats and composite aircraft lack the shielding of a metal cocoon. The same consideration applies to portable operation. The current 706mkiiG has poor EMC in the control head and microphone input. Quite a few hams have reported EMC problems. It's time for ICOM to stop stonewalling this issue.
  • A noise blanker that works well on AM. Let's face it, AM will be around long after the 706.
  • A medium AM bandwidth, possibly with an optional 6 kHz filter. Continuing improvements in digital logic will allow small radios to use DSP functions now seen on the 756 Pro series. The only two selectivity choices for AM are too wide and too narrow.
  • Higher sensitivity on LW and MW below 1800 kHz when used with short antennas.
  • Higher selectivity and lower capture ratio 88-108 MHz WFM.
  • Better bass response to the headphone jack.
  • Improved front end selectivity and dynamic range 88-136 mHz. A tracked front end is needed for FM broadcast.
  • A 118-148 MHz bandpass might be sufficient for the air band and 2 meters.
  • Continuous receive coverage up to 470 mHz.
  • DTMF Microphone
  • A higher switching threshold on the PTT line, e.g., 3 volts.
  • Easily adjustable panel backlight intensity and/or photocell.
  • Better transmitter filtering and shielding to prevent transmitting on 2 and 3/4 meter from disrupting VHF navigation 108-118 MHz.
  • Voltmeter reading of power input voltage.
  • The 220 MHz ham band and an ability to receive 220-400 MHz would be a plus.


    The first radio related mod to N2469R was the installation of an antenna on the belly. A camera port located on the belly provided a good venue for the antenna.

    The ICOM IC-706 MKIIg battery cable had its one and only fuse in the ground lead. I moved the fuse to the positive lead next to the battery. This avoided the possibility of an unfused short between the positive cable and the airframe causing a fire or explosion.

    For the temporary installation the ICOM was connected directly to the battery. The 706 MKIIg draws a small current even when switched off.

    After satisfactory operation was verified with the temporary HF antenna a 337 form was submitted for installation of the AH4 tuner and antenna. This installation cuts down on the amount of RF in the cabin, allowing some of the EMC hacks described above to be removed. The ICOM battery cable was moved from the battery to the master contactor (relay). The 706 is completely powered down when the master is off.

    A 2 meter or 440 Handheld Will Do

    For penguin VHF Hams, a 100 foot tower on top of a 500 foot hill is an incredible QTH. Up that a few thousand feet and you don't need tropo to get out. A 2 meter handheld gets out just fine. After my CFII was unable to call his wife with his TDMA cell phone, I was able to make a 2 meter autopatch for him.

    If you connect the handheld to a spare air band antenna the outside antenna's clear view may more than offset the higher SWR. You can make an autopatch from halfway across the state. Cockpit noise affects ham handhelds, so consider a headset adapter.

    Vertex Spirit 700 All In One

    Recently a fellow ham pilot purchased one of these $400 wonders. It covers FM broadcast, air nav, air comms, 2 meters, and NOAA weather.

    When Phil showed me his radio, I was impressed but worried the wide frequency range might make the radio susceptible to cross mod from commercial stations. This is apparently the case. He couldn't receive the Newberg VOR from Stark's Twin Oaks (7S3) 4.8 nm away with the VOR cone in plain sight. A second unit was also unsatisfactory. He was also disappointed that the Vertex could not navigate a localizer despite advertising the ability to tune the localizer frequencies. After much complaining he was allowed to return the radio to the dealer. The radio might be fine in the sticks but not near a city. This radio has been discontinued.