US Military Air To Ground Communications System
The air to ground communications project looks deceptivly simple at first blush. However there were multiple problems that had to be solved.
The first issue was a radio system that was so old that it looked like it came from WW2. However, in reality it came from the early stages of the Vietnam war.
The second problem was the cabling was put through a hole cut in the window, with duct tape used to keep the weather out.
There was no electrical grounding, or lightning arrestors in line with the coax. On the roof where all the antennas are mounted, they were placed on one pedestal.
This created a problem every time some one would key up (transmit) with one radio, the transmitted signal would "desense" or wipe out the other two radio's receivers.
Thus all receving abilty was lost. Another problem we found once we became involve with the project was that the antennas were not maintained.
Thus when we attempting to disconnect them from the coax, the antenna connector crumbled in our hands. As you can see, we had our work cut out for us.
NEW AIR TO GROUND RADIOS
Malloy Communications replaced the existing communications system with new GENERAL DYNAMICS URC-200 radios, that has an extended frequency range.
With the additional "card" the radio will transmit from 30 MHz to 400 MHz continous,
with the exception of the FM broadcast frequencies of 88 to 108 MHz.
One of the problems with the radios is the antenna connections are on the front panel. There are two connectors.
A panel mounted female N-connector for the frequency range of 30 to 88 MHz, and the panel mount BNC for 115 to 400 MHz.
This could have created quite a problem for the radio operator, who must contend with mic wires, and antenna coax all with-in close proximity to each other.
The way that Malloy Communications solved this problem was to design a patch panel with right angle connectors with short coax jumpers to re-direct the coax attached to the front of the radio over to the left.
From there, another right angle connector was used to re-direct the radio signal to the actual coax mounted behind the patch panel.
This also allowed the radio operator to disconnect one radio from a particular antenna and re-connect it to another antenna if the need arose. Thus flexibility was of paramount importance.
On the other side of the radio, the mic's were mounted at angles to each other to keep the mic chord from tangling with other mic chords, when in use.
You should note, there are 3 radios stacked on top of each other in this configuration, each scanning certain Government air to ground communications frequencies at any given moment.
The original coax was basically a ball of coax running helter skelter from inside the radio room out through a hole in the window (covered with duct tape).
We replaced all of that, and ordered 5/8" helix from TESSCO.com.
We had them TDR the lines to make sure there were no problems at the connector ends.
We then built our own "railroad tracks" to mount the helix runs on, to keep them from just laying on the roof.
At the antenna end we used another TESSCO product to keep the weather away from the N-Connectors. The joke on me is that I only came out to have my picture taken, then went running back into the Air Conditioning! It might be true.
The antenna requirements were quite demanding.
I cannot give specific frequencies, however in this particular application, the primary frequency was between 135 to 138 MHz.
Their back up was at 380+ MHz. One antenna had to provide a reasonable VSWR for that wide of a frequency demand!!
That is an almost impossible task.
However, there are two classes of antennas that are able to meet those frequency requirements with out blowing up the radios with high VSWR.
One is the DISCONE, the other is the BICONICAL. These antennas are in the same family of antennas and perform quite well.
I elected to design the BICONICAL for this application. I wanted to insure a true vertical polarizated signal.
The technical literature states that the DISCONE is also vertically polarized, but I have found that it seems to favor horizontal polarization,
when conducting field strength measurements with both antennas.
Specifically, using the DISCONE as a receiving antenna we rotated the antenna from horizontal to vertical,
while receiving a carrier wave.
The S-Meter (LED bar-graph) deflected more when the the receiving DISCONE antenna was in the horizontal position
than the vertical.
Thus, the BICONICAL won out. Also, you should not think that any broadbanded antenna provide flat VSWR's.
The VSWR curve goes up and down on any broadbanded antenna.
With the BICONICAL, changing the angle of the radial elements will change the VSWR curve.
Thus if a customer wishes a particular set of frequencies the angles (machining process) must be experimented with,
to find the proper VSWR curve for that particular application.
Malloy Communications does not mass produce these antennas.
We design the antenna for a paticular application only.
They make excellent scanner antennas, but would be too expensive for the average scanner listener.
If you would like to see an Excel spread sheet on some of the expermental VSWR work done with the BICONICAL, please feel free to email.
The low band VHF antenna (looks like a paper clip) was purchased from TESSCO.
These types of antennas produce about a 6 MHz bandwidth (2:1 to 2:1).
The problem with these types of antennas they are not omni-directional.
The tower is part of the loading process and acts as a reflecting element.
The radiation pattern is about 180 degrees in their current mounted position.
Because the customer wanted two of these antennas, we mounted one pointing to the west (radiating South and North) and the other phased to the east (also radiating South and North).
In effect, between two radios scanning their low band VHF frequencies they had full 360 degree coverage with a 3 dB gain antenna,
amplifying a 100 watt signal. Not bad.
ANENOMETER ROOF MOUNTING