Consulting business

Recently I decided to advertise RF and Antenna Consulting from my home in Bend, Oregon. There are many Antenna users in Central Oregon including the telecommunications providers, aircraft, and cell phones. One thing we're famous for in Central Oregon is lousy reception and that is largely do to the limited tower locations, crowded bands, and geographical situation.

Many or the providers of Internet service in the area are non-technical. I hope to provide a source of good technical advice and perhaps to become the consulting engineer to several competitors, so I will have access to information which could help avoid possible conflicts on the towers.

I have designed test antennas for SprintPCS and tested them on their outdoor test range in Kansas. I have performed field microwave radio tests that taught me a lot about reflections and call drop out. I was the Chief Engineer of Rangestar Antennas at the time.

Aircraft Antenna patterns

Aircraft antennas are divided by frequency range, speed/type of aircraft, visible/covert, active or resonant, shape - blade/dish or other, and ESD resistance.

An aircraft can have an antenna array that includes a radar dish in the nose, HF wire, VHF and UHF blades, steered microwave data dish in the tail or on top of fuselage for satellite. Some aircraft have huge arrays of VHF and UHF antennas under the belly and wings, and some have SAR antennas on the belly or in "chipmunk cheeks" near the nose.

What they all have in common is the requirement to receive clean RF signals uncluttered by the aircraft's shape. velocity, or attitude. Active antennas of course cannot transmit, but for all resonant antennas the principle of antenna reciprocity means that they will transmit and receive with exactly the same power lobes in the antenna pattern.

The skin of an aircraft is imperfect. It is made of riveted plates of aluminum which corrode over time. The gap (negative space) between the rivets create spaces that are antennas just the same as if they were wires or positive space structures. The act of semi-conducting, meaning in this case "partial conduction through active regions", makes the skin of the aircraft a component of the radio system. I have actually measured the 2nd and 3rd order Inter-modulation Products of aluminum ground planes built to simulate the skin of aircraft. While the IMs are low power, they pose a problem for reconnaissance aircraft by creating phantom signals.

Since an aircraft is a conductive body, it is an antenna with it's own resonance frequencies. Since all antennas have to (sub-ohm) bonded to the skin (Ground plane) of the aircraft, the signals getting from the antenna to the radio are also modulated by the fluctuating current on the skin of the aircraft. This source of extraneous RF energy that can create IMs in the radio.

Do to the counter modulation effects of the aircraft skin, which vary with the attitude of the aircraft to the signal source, the pure sinusoidal waves that would be detected by a ground station are misshaped when detected by the same equipment mounted on an aircraft. The degree of misshape varies in a fixed manner relative to source position so the actual signals can be offset by having the offset stored in memory and multiplying the received signal by the offset (after digitisation).

Velocity affects antenna design to a huge extent. Antennas for low speed aircraft need not be extremely streamlined, rugged, or ESD resistant. Antennas for jets must be ruggedised and tested to loads from all angles as well as extreme vibration, as a loose antenna could damage a plane. The faster an aircraft moves, the more static electricity is generated. Military antennas and radomes have a coat of slightly conductive paint over them which carries the static charges to the body of the aircraft.

Some antennas are embedded in aircraft. Antennas on composite wing aircraft are subject to destruction by lightening. Some antennas are covert and extend electrically after a plane is air born.

Active antennas are used for receive only applications. They are non-resonant and can have 150-1 band widths. A typical high quality resonant antenna will get 6% to 10% bandwidth at 3db points. An active antenna may have power carried to it by the center conductor of the coax, in which case the RF and Power are separated at each end of the cable by an appropriate network.

triple use circuit cards

One of the common requests is RF, Digital and and an antenna on the same printed circuit card. Requirement one is to keep the digital out of the RF, and requirement two is to get an antenna to work well on the same board.

Digital signals need to be contained. Any lost energy will seen as system noise and thats a bad thing. The technique I used on boards where noise and loss was critical was a 5 layer board. The top, middle and bottom were ground planes. Layers 2 and 4 were used for signals - one side for digital and one side for RF. The edges of my boards were plated from top to bottom creating a very low impedance bond between all ground layers.

You want a dielectric constant around 2 o 2.5 to help contain the energy of the signals on the traces.
BUT
Antennas want to launch waves and cannot be built on the same substrate. They want a substrate that allows the signals to escape easily and to this end, antennas rarely want ground plane under them.

Antennas can be mounted on printed circuit cards, especially on the edge and raised just a bit so there is air between the antenna and the circuit card.

Antennas on high dielectric substrate simply will not work, the energy will not launch waves effectively.

Antennas can be built on FR4 if the frequency is low, simply by using one layer only in the antenna area.
Jim

Ground

First off - the most important element of any Antenna System is the Ground.
I have walked into interviews and said, "Without you even telling me anything about your product, I can tell you that you probably have grounding problems" and get knowing knods of heads.

Ground is a large polished continuous piece of highly conductive metal of reasonable thickness, atleast one wavelength in Radias with the antenna situated in the middle. Anything less than this is a compromised ground which means imperfect antenna propogation patterns, uneven gain distribution, and possible center frequency shift.

A wire is not a microwave ground or even an RF ground. A ground should be unbroken, however the reality is that we often get a corner of an over populated RF/Digital pc board with no regard for the antennas function. In this case try to place the antenna as close to the middle of the board as possible and away from other tall components. (Note: some antennas will only work on the edge of the board.)

Welcome

Hi People,
Welcome to this new blog about Antenna Design and wave propogation. You will find information here on the design, manufacturing, and testing of modern recconaisance and communication antennas.
Jim