Madsen: Beating RF Communications Jammers
 
Date: Thu, 1 Nov 2007 22:55:34 -0500 (CDT)

http://www.waynemadsenreport.com/articles/20070411_71

publication date: Apr 11, 2007
September 6, 2005 -- For communications technicians in New Orleans
and Gulf Coast region. How to beat the communications jammers.
(Thanks to "M"):

BEATING THE COMM JAMMERS

There are methods to greatly reduce the effects of said jamming.

This has to be, in it's nature, a bit technical to get you to
understand how to get there from here.

There are a couple of ways to raise the ratio of the desired signal
yet attenuate the offending signal.

The most desired is of course using a very directional antenna aimed
at the desired station, but in a typical field emergency situation,
the operators are likely using a dipole antenna strung between
whatever supports they have available.

For the broadband or direct on operating frequency jamming, add a
second antenna broadside to the offending transmitter (or suspected
area), take the energy from this antenna and using coaxial cable
of certain length (this will depend on the distance from the extra
antenna to the original antenna, relative to the jamming station),
the cable is cut to a length that, when both cables are brought
into a "T" connector into the radio, the signal from the jamming
transmitter is brought into the circuit at 180 degrees out of phase
to the signal being also received at the original antenna from the
jamming transmitter, canceling out to a great degree, the energy
from the jamming transmitter.

Coaxial lines have a certain velocity factor compared to free space,
that is to say the signal velocity is slower in the cable media.
The older plastic coax has ~.66 speed where the more modern foam
coax has ~.88 so using this data, the spacing from the original to
the canceling antenna, one can calculate what length of phase delay
line to use in obtaining this 180 degree phase cancellation method.

Though today's tests are not as tough as they used to be, the hams
out there will know how to do this if it is brought to their attention
and will know how to calculate, providing they remember this method
in their frustration.

As far as the IF jamming issue goes, to be effective, for a high
quality communications receiver, it takes a huge amount of power
or to be very close in to the radio site. For a normal AM table
radio, that is not too hard to jam. You have usually a single
conversion receiver (cheap). If the IF frequency is .455 MHz, the
local oscillator will run at either .455 MHz above or below the
desired signal. The desired signal is mixed with the local injection
signal and you get the sum and difference frequencies.

So, you have 3 frequencies that the radio can detect, the "front
end" is tuned to amplify greater at the desired frequency with a
tracking section of the tuning cap, the other section running the
local oscillator.

Thus, the radio can hear, in orders of relative strength, the desired
frequency, the image frequency (one being the sum, the other being
the difference, of the mixing from the Local Oscillator and the IF
frequency. You can also hear signal at twice the .455 plus the
products.

As the radio was built to select by tuning of the amplifier with
tracking tuning, the greater gain is on the desired frequency, yet
there is in the simple design, not enough rejection to ignore a
strong enough image signal but there is further rejection at the
final IF frequency, so that is the less prone to unwanted signal
but still possible.  If one were trying to jam reception in the AM
broadcast band or on a single conversion receiver, yet did not want
it to be too obvious and jam the actual frequency of the broadcast
band transmitter, one would have two options. First, generate a
signal on the image frequency of about 3 or 4 times greater and to
the detector stage, it will win out over the desired frequency.

Everybody is looking at the AM broadcast frequency, hears no beat
notes or jamming signal, assumes it is a local radio problem. Not
many will go looking at the image frequency, not your typical
consumer, that is.

In this situation, as the radio uses a ferrite bar or "loopstick"
antenna internal, it has quite a directional nature, in fact if you
were out in the woods lost and knew where a local radio station
transmitter site was, on the AM band, you could rotate the radio
and hear a distinctive signal null off the ends of the loopstick
antenna, try it. You get one null and the loopstick has 2 ends, so
you get 2 directions. If you are near a clearing, make a line to
two objects you can see from another location and remember them.

Then move position as far away within view and take another null,
make another line to landmarks. The two landmarks closest together
is the way home, the two further apart is the wrong way.

Handy for finding noisy electrical devices around the house, too.
So, turning the radio to null the offending or jamming signal reduces
the signal and raises the strength of the desired signal, as long
as they are more than a few degrees off a 180 degree line from where
you are at. Of course you can null the desired station and peak on
the jammer, too, your choice.

For the signal directly on the .455 KC IF, which can be other than
that but this is a common one, it will have to be many times stronger
than the desired signal. This raises a power issue I will address
later.

For a communications class of receiver, there are dual or triple
conversion radios. This is done to help beat the image signal problem
as described with the cheap AM table radio.

You have a high IF, often at 10.7 MHz, where having the RF stage
fairly selective at the desired frequency will admit this yet with
enough bandwidth to easily pass all the modulation and sidebands.

So, the chain would be, amplify at the desired frequency, local
oscillator 10.7 MHz above or below, pass the signal to the mixer
but now with enough selectivity to attenuate the left over product,
sum or difference.

Here you still have enough bandwidth but greatly reduced one potential
image frequency.  Then, another mixer stage, this one at say .455
MHz. You end up with the signal converted to a final IF of .455 MHz
where it can go through a filter for upper or lower side bands,
etc. here you get real selectivity.

My point, if you have a dual conversion receiver and remember some
are triple conversion, getting that .455 MHz signal into the shielded
stages with all the prior filtering at each conversion takes a
tremendous power at even modest distances.

Bothering say a 23 MHz IF, a 10.7 MHz IF and a .455 MHz IF is very
hard.

Consider these factors: If it were a .455 IF and transmitter AND
if all the receivers were operated on commercial power lines, some
of the RF can ride in with the power lines. However, here we are
working with battery or generator local power supplies so the power
line acting as a "carrier current" type broadcast assist is not
happening. By the way, speaking of carrier current, in times when
the power grid is normal, this is another method of getting a lot
of signal to the radio one places the RF carrier directly on the
power grid and the energy is distributed strongly great distances
on the grid, yet is hardly heard when less than a couple of hundred
feet away from the wires.

All the radios connected to the grid hear the signal very strong
yet, on a battery, the same radio, once away from the power lines,
can hardly hear the signal. Also makes it hard to locate the
transmitter with simple equipment.

Considering the situation of the power grid, that is not too likely.
However, if you are having a direct .455 MHz direct IF blocking
problem, or the next up common IF of 10.7 MHz, if the signal is
having a overload or blocking effect on the receiver, the ship out
to sea may be too far for real effect.

Look closer, an ideal placement would be the local AM radio station
tower, tuned with a series coil, it will have a good signal output
and also has the required ground wire radial system built in beneath
it, for efficient energy and ground wave transfer.

As the problem described seemed very local to the area, somebody
may have hung a high power 455 kHz transmitter and / or 10.7 MHz
transmitter very local.

Even with the station operating, it is easily possible to inject
another tuned circuit to keep the two transmitters from interacting
or back feeding to each other and sharing the same tower(s) and
ground system(s).

I would have no problem pulling this off very low budget so they
would be able to do this easily, too.

How to block the IF busters; Place an in-line (in the coax cable
path) band pass filter so that the common Low IF frequencies are
greatly attenuated, between the antenna and radio. If the 10.7 is
under attack, do the same or both.

For a ship bound system to be as effective at .455 kHz as a local
radio tower, it is going to require a balloon lifted long wire and
several times the power, if the broadcast station tower is closer.

Yes, the ship will have a decent ground counterpoise but unless it
has a very long antenna, it will have to make that up in a vertical
antenna resonated with a coil. At this frequency, you are talking
maybe a 10 percent efficient system, plus inverse distance losses
depending how far out the ship is. Megawatts would be needed.

A lot of megawatts. A tube type CQK-650 can do 1.77 Mw output driven
with as little as 900 watts. VOA uses them, other international
broadcasters, too. We used to rebuild that type tube and it's brother
the CQK-450 so I know who uses them. (openly).

Shifting operating frequency for the radio nets is one option if
everybody gets the word on time. Spread spectrum is effective but
the ECM people on the other side will see that. I am within reach
of a spectrum analyzer from 10 kHz to 22 GHz and trust me, if it's
there, it can be seen, isolated and locked onto.

By the time these countermeasures are brought to the field teams
attention, whatever they are trying to hide and block may well be
over. But maybe not.

The last measure is diversity transmission and reception.

Two or more transmitters and time delay on one of them (or more)
(the rules say, unless you are broadcasting code practice) you can't
put the same information on 2 transmitters, however, if you time
delay one, well, gee, it's not the same information at the same
time, is it...then time delay the second receiver (or more) then
sync the information back again, minus signal fades, you can recover
more data.

Gov't did radio teletype that way for decades, before computers.

If the guys out there are real newbies, they may not be able to rig
that up without a lot of gear.

***

Additional info:

For on-frequency interference, there is a commercial device that
works quite well for nulling out the unwanted signal and/or enhancing
the desired signal.  This model has its own antenna, eliminating
the necessity of erecting a separate "noise antenna" and building
a phase delay line, etc. as described above.  Amplitude and phase
are both adjustable from the front panel of this box, allowing the
operator to fine-tune by ear in real time, greatly improving the
chances of successful attenuation of the interference.

******************************************

"Team Bush" still refusing international aid. Russian rescue crews
on four cargo planes with helicopters on board sit idle at an airport
near Moscow waiting for green light, Cuba has 1500 doctors with 26
tons of medical supplies and Bush is refusing them entry to U.S.,
Venezuelan disaster rescue teams wait for a "go," Dominican Republic
crews with hurricane recovery experience wait and wait and wait.
It's the same scene at airports around the world. Meanwhile, FEMA
turned back 8 buses from Washington, DC that were to bring 400
evacuees to the DC Armory which has been stockpiled with just about
every need. The Bush regime has finally reached the crescendo of
evil.

And why is FEMA so incompetent? It was turned into a political
patronage agency by Bush for Bush's campaign lickspittles. This
from a disaster recovery specialist with inside contacts at FEMA:

Mike Brown was Joe Allbaugh's college roommate. (Allbaugh was Bush's
first FEMA director). Allbaugh is now a consultant who makes tons
of money for greasing the skids for companies to get business with
the federal government in Iraq and with FEMA. Note to Federal law
enforcement. You might want to investigate the relationship between
Allbaugh and IEM, Dewberry, and URS -- the companies that took
$500,000 to develop a "Catastrophic Hurricane Disaster Plan for New
Orleans & Southeast Louisiana," a plan they never delivered and a
project for they fraudulently included James Lee Witt Associates
as one of their team partners. The sleazy revolving door spewing
out money spins very fast with the Bush administration.

Mike Brown had absolutely no experience in disaster management.
When Allbaugh became director in January, 2001, he politically
cleansed FEMA of anyone who was associated with outgoing director
James Lee Witt, who had done an excellent during the Clinton
administration. Allbaugh was charged with cleaning out of FEMA
because of the neocon desire to rid the Federal bureaucracy of FEMA
and replace it with charitable giving as a means of disaster relief.

So Allbaugh's first priority was to gut FEMA.  This became an even
greater priority after the Democrats (especially Joe Lieberman)
forced Bush to create a Department of Homeland Security.  FEMA was
rolled into DHS.  Allbaugh was caught by the Government Accountability
Office (GAO) using the FEMA Boeing 737 aircraft for private purposes,
including numerous personal trips to Florida and political trips
around the country. The Republican National Committee reimbursed
FEMA for the political junkets, but the embarrassing trips hit the
pages of the New York Times and Allbaugh chose at that time to leave
and set up his own consulting business.  He left without any pressure
from the White House.

In early 2001, the International Arabian Horse Association fired
current FEMA director Mike Brown as its executive secretary after
a non-productive period of employment.  Admittedly, Mike Brown had
spent a large part of 2000 campaigning for Bush rather than doing
his job, but the bottom line for the horse association was that
Brown neglected his duties and day to day responsibilities.

Brown was lucky. His old college roommate, Allbaugh, appointed Brown
as his number 2 within days of his firing by the horse association.
This cozy relationship was noted throughout  the Allbaugh period.
Upon Allbaugh's departure, Brown was designated by Bush to succeed
him as FEMA director. He is called "Brownie" by Bush.

"Brownie" has few supporters in FEMA.  In fact, the general commentary
is that he lacks basic organizational and management skills and he
is a bullshit artist, not a leader.