Airborne Crossband Repeater Project
In the summer of 2008, several experiments and flights were made to establish if a crossband repeater could be utilized to extend the range of Clallam County's emergency communications capability. The idea being that if an airplane carrying a Cross Band Repeater (CBR) flying around 5,000 feet above the West end of lake Crescent could cover all points in the county, then we would have no loss of contact with any area within our responsibility. Antenna used would be either a window mounted VHF/UHF rubber ducky or the aircrafts 130 Mhz aviation antenna shared with other equipment through an RF splitter. The objective was simple, carry the portable repeater onboard, connect antenna through the splitter, connect 12 volt power, place in cross band mode with frequencies pre-established and off you go.
The final test was a success with certain reservations considered.
that reliable communications could be maintained out to about 40 miles
from the airplane.
mounted aviation antenna was superior in performance even with great
inefficiencies of mismatch. The UHF performance was the weakest
link but we were able to make contact to all points. It should be
that ground stations need to be more than simple handhelds unless they
are within 25 miles of the aircraft. Were
talking about established base stations or mobiles for effective
communications at the fringes. Increase in intermod products was
observed particularly close to the airport when using the shared
Detailed flight information is
available along with sound recordings
for those wishing details. Anyone attempting this
should be sure and understand FAA limitations and what you can and
do aboard small aircraft. Concern and respect should
be made to navigation equipment, particularly ILS.
Although these test were done with airborne use in mind, the system
can and most often will be used for terrestrial applications.
the more common usage. Lets face it, it will be quite a disaster
to force the flight of an aircraft in times of distress but it
certainly has happened in the past.
Antennas, weight, bulk and power supplies are unique in aircraft and
have a set
of rules that must be followed but its easy to take the repeater
and apply it for terrestrial use since its already packaged and easy to
Often, the antenna can be a
rooftop mount on a vehicle or a more elaborate one for emergency
portable use such as found with a WINLINK station. When using a
repeater the antenna must be tuned as a dual band unit. In other
words, if the two bands used say are 144 Mhz and 440 Mhz, the antenna
must be a reasonable radiator on both bands. Otherwise, loss of
efficiency will ocurr plus possible damage to the radio due to high
VSWR. Battery power can be supplied from the vehicle or a
separate 12 VDC source which is preferred.
The nice thing about modern crossband units is they can handle simplex, duplex channels along with CTCSS tones as needed by simple programming. You can interface and extend the range of either a ground based repeater or simplex channels. In an emergency, the CBR could be programmed on the spot to satisfy a unique situation where radio coverage has been lost from a geographical area. The intent is to reestablish that coverage with the CBR on a temporary basis. You can even program for terrestrial to terrestrial repeater linking but its not encouraged because of the complex audio path the signals have to pass through. Distortion is often increased to marginal levels.
The CBR must have tight control of power output
because it will work twice as hard as a normal station. Most
radio's including those with a crossband capability are intended for
intermittant use. If you run at higher
duty cycle you run the risk of damaging it. Most operators limit
power to 10 watts maximum. For public Service events, 5 watts
is more than adequate.
The FCC defines requirements the control operator must meet if
running a cross band repeater:
Most CBR's manufactured today have minimal "hooks" or provisions for meeting FCC rules in CBR mode. In short, this means you'll have no provision for ID and no remote means of disabling it. You'll have to provide these yourself.
Figure 3 shows a diagram of how I solved this. Being an enthusiast of foxhunting, solving the IDer issue was relatively easy. I build small radio transmitters with voice ID that are used in our "bunny" hunts. They're highly portable with self contained battery and can be set for multiple frequencies. They don't have to be physically close to the CBR but need to be within radio range. Each transmits once every 10 minutes and is programmed by "dip" switches with built in microphone for audio recording.
Also, borrowing from my transmitter hunting toolbox is the DTMF decoder. Hidden transmitters are often controlled remotely by the use of DTMF tones transmitted to the fox receiver. The controller decodes these tones and then determines the action needed. The most important one for this application is to disable the transmitters. That function is already built into the software. Unlike the IDer, the DTMF controller must be physically close to the CBR since it has to borrow audio and disable PTT circuit in the repeater.
When using it in an airborne mode, neither the IDer 's nor the DTMF decoder is needed since it would be assumed that the passenger would be a licensed amateur radio operator and in full control of the radio at all times. He would personally ID the transmitters and cut them off if problems were to develop.
One of the best radio's for doing cross band operation is the Yaesu FT8900-R. Its a quad bander that has 29, 50, 144 and 440 Mhz capability with cross band operation on any two bands. All you need for compatability is a dual band antenna that matches the frequencies you wish to use. This radio operates in a bi-directional mode so it can transmit on UHF and receive on VHF as well as transmit on VHF and receive on UHF.
This repeater has three uses that come to mind with the most likely listed first:
The nice thing about CBR's is that their operation is nearly
identical to normal full duplex repeaters. About the only thing
you have to learn is the switch from VHF to UHF (or UHF to VHF) when
you stop transmitting might take a few 10's of milliseconds longer.
Operators just need to hesitate a little longer between transmissions.
On the otherhand, simplex repeaters, the other type of portable repeater, have a steep learning curve for the uninitiated. Simplex units rebroadcast the previous transmission on the same channel and cause considerable confusion. Not the kind of thing you need in an emergency involving people that have not used one before. Most people that have tried simplex repeaters end up dropping them for this reason.
The photo shows the complete terrestrial version of the repeater. Only the dark green "ammo can" in the upper right hand corner which contains the radio would be needed for the airborne version since it would obtain power off the aircraft and use the outside mounted antenna. A few extra connectors and adapters should be included to be sure their are no last minute surprizes. The IDer would not be needed as long as the passenger ID's the repeater per FCC rules. Can't get much simplier than this one box implementation plus cables and microphone.
The rest of the equipment is for the ground based
implementation. Included is the IDer in the small "ammo can", the
white "stick" which is a dual band "J" pole antenna, the gray breakdown
stand, and the 75 AHR 12 volt DC battery. This configuration can
be placed on a hilltop if a weather covering is provided for the
battery. The rest of the equipment is already in waterproof
containers. On the otherhand, it might not be a good idea to
simply put it out in the woods because the radio is expensive. I
have reached a point that if it can't be locked up in a vehicle or on
someones property, its not worth the risk. Individual situations
must be considered.
Transmitters for foxhunting are much less expensive and we often
make the antennas very stealthy......they look like tree limbs even at
close range. The transmitter and controller are covered in
biomass so its virtually impossible to find them by accident.
Only good foxhunters have a chance.
Neil Robin, WA7NBF
Port Angeles, Wa.
Neil A. Robin, WA7NBF