TSCM Antenna Methods and Protocols
Gain parameters are commonly used to model the performance of both and transmit and receive antenna, but Antenna Factors are normally only used to model the performance of transmit antenna (which are more precise than receive only units).
While it may seem bizarre to specify AF for a receive antenna it starts to make sense when you are using calibrated EMC/EMI antennas for all TSCM procedures.
Of course Antenna Factors increase with frequency (as does gain), and remember that Antenna Factor is different from Transmit Antenna Factor (so be careful). (Antenna Factor is calculated as AF = Electric Field/Voltage applied to Antenna and is normally expressed in dB).
Remember that while Gain is commonly used to delineate the directional response, AF will describe how "clean" the antenna is. When performing TSCM we want the measurements to be as "clean" as possible as we are working only slightly above the noise floor and have to fight for even a fraction of a dB.
The high threat VHF band should be checked using an antenna factor of at least 10 dB 1/m for signals between 20 MHz and 300 MHz. Antenna gain should be between unity and 4 dB over this range. A bi-conical or discone works well in this band.
Antenna factors for frequencies above 300 MHz should be between (at least) 15 and 20 dB 1/m. Antenna gain should be over 4-8 dB from 300 MHz to 1 GHz. Log periodic antenna work well in this band, but for optimal results use a spiral log antenna.
At frequencies above 900 MHz it is desirable to use an antenna which offers a factor of at least 25 dB 1/m, and 30 dB 1/m above 2 GHz. Antenna gain should be at least 8-10 dB above 900 MHz (a gain of at least 15 dB is preferred). A gain of at least 15 dB is recommended between 1.2 GHz and 3 GHz.
From 3 GHz to 12 GHz an antenna factor of between 30 to 40 dB 1/m is good, but anything above 12 GHz calls for at least 40 dB 1/m of antenna factor, or it will be virtually impossible to detect the signal. Antenna gain should be at least 8 dB over this range. A gain of at least 20 dB is recommended between 3 GHz to 6 GHz, and at least 30 dB up to 12 GHz.
Above 12 GHz antenna factors of at least 35 dB 1/m must be contemplated. A gain of at least 50 dB should be used when dealing with frequencies in this range.
The antenna being used must be operated in both a horizontal and vertically polarized position, and in a 45-degree diagonal position for optimal signal detection.
Left and Right Handed Polarized signals should be checked between 900 MHz and 12 GHz using an antenna factor of at least 25 dB 1/m and a gain of 2-4 dB. Remember that you will get a polarization offset loss figure that must be taken into consideration when using a linear polarized antenna.
The number of antenna azimuth and elevation positions used will be directly related to the gain of the specific antenna.
A good rule of thumb is that in a 10-meter by 10-meter square room in an office environment to use at least 16 steps (22.5 degrees) along each axis for every 10 dB of antenna gain. For a typical log periodic antenna (with 5 dB of gain) this would lead to 64 measurement positions for each polarization for a aggregate of 320 positions (assuming horizontal, vertical, diagonal, LHCP, and RHCP polarization). Naturally, a small computer controlled antenna positioning system is most helpful.
Here are some of the better solutions to antenna requirements that the author personally uses during TSCM work:
- A 50 kHz active loop for magnetic fields, 100 kHz low pass filter,
and a 40+ dB LNA (critical for finding video cameras, and lines being
used to carry video signals). A 60+ dB amplifier is perfect for this,
but you're going to have to knock down the 500 kHz AM BCB band by about 80-100
dB, or you risk saturating the instrument.
- A 30 Hz to 50 MHz (to 110 MHz optional) high dynamic range active
rod antenna with a removable ground radials or 24 inch plate. Lean
towards antenna elements that can be broken down and transported in a
briefcase. Battery powered is a big plus, and the antenna has to have
a very wide dynamic range. Also, you will need to have filters to put
between the antenna and the instrument or you may get Intermod that
will drive you nuts. The author prefers the ARA RAM-110/B
- A 9 kHz to 30 MHz (to 50 MHz is best) active loop antenna, a nice
24 inch should be fine.
- A quality biconical antenna with balun and Delrin spacer blocks.
It has to have the impedance bar in the "birdcages," and avoid the
portable elements as you get poor results below 50 MHz. You will need
a really good LNA you can dedicate to this antenna, and the author would
recommend the Sonoma 310. The author would also recommend that you procure a
set of bandpass filters (with really low insertion loss) so you can
keep the Intermod to a bare minimum. The filters are important as the
low VHF TV channels and FM BCB can play havoc with the LNA. A
firewall 30 MHz highpass filter will also be helpful. This antenna
is best used from 88 MHz to 300 MHz. Remember that with a biconical
that you cannot have metal interfering with the pattern of the
elements (hence the Delrin spacers)
- An equiangular, spiral log periodic antenna (it looks
like a big cone) that covers 200 MHz to 1 GHz, a 200 MHz high pass
filter, and another Sonoma 310 LNA. Of course, you will also need some
bandpass filters as well, or you risk saturating the spectrum
analyzer, and getting poor sensitivity. Also, the antenna is best
used from 300 MHz to 1 GHz.
- A 450 MHz to 2.7 GHz discone is also helpful, and the author would
recommend the Ramsey unit, but get the passive version and add your
own amplifier. While the ramsey unit is not a calibrated labratory grade unit ,
it does provide a quick and inexpensive solution.
- You may also find it helpful to procure a lower band discone as
well. The Diamond D-130J, Icom AH7000, and Radio Shack 20-043 discone
antennas are all good, and very inexpensive.
- Things start to get a bit dicey above 800 MHz (at least here in
the US). The big problems are the 800 MHz cellular bands, the pager
bands, and the 1.8 GHz PCS bands. With this in mind you will need a
dozen of so band pass filters, and a half dozen band reject filters.
You can use the discone to detect the presence of these signal, and the
spiral log to DF it. However, watch out for cellular/PCS phone being
used as an eavesdropping device (mind the remote channels).
- Above 2 GHz we can start using high gain antenna, and this authors
personal preference to use calibrated standard gain horns instead
of broadband horns for sweeps. The broadband horns are good for
general EMI/EMC work, but the extra gain and flatness of the standard
gain units make them more attractive for TSCM work. Of course, each
horn will need a sharp bandpass filter and a high gain LNA. The author leans toward a
-80 dB out of band filter, and a 54 dB LNA mounted directly on the
horn. This works well from 2 GHz to about 12.4 GHz.
- On the extremely high threat 2.4 GHz and 5.8 GHz bands the author also prefers to use a patch antenna (16-20 dB) to find the signal, a narrow band inter-digital filter, and a 32-54 dB LNA. This author then splits the signal and use half of it to drive a 90 dB RSSI indicator which makes it very easy to DF the offending signals. The other half goes through a detector, and then to a pair of headphones (or O'scope). The author can bypass the splitter and drop the signal right to a SA if he need to. The author also uses a similar system without the splitter to drive a signal into a microwave spectrum analyser. Once the signal is located then, the author switches to calibrated antennas for the actual measurments.
Above 2 GHz, but below 12.4 GHz you want about 45-70 dB of gain. This breaks down to 16 dB from horn gain, and 40-54 dB from the amplifier. Above 8.2 GHz you should try to stay with a dish system with about 30 dB of gain, and an amplifier with at least 40 dB of low noise gain (54 dB is a good number).
You may also find it handy to have a half dozen small antenna's and some method of attaching them to things like edges of desks. These are also great for hand-held use, and for general purpose scanner antennas. The Radio Shack 20-006, and 20-023 make a good combination, and you can attach it to a nice clamp you pick up at Home Depot or Lowes.
If you don't yet use the Sonoma Instrument you should call them and ask to speak with Vladan Temer, his phone number is (707) 542-8569.
For those of you not familiar with EMCO, they can be reached at: (512) 531-6400, and this author would strongly encourage you to obtain several of their printed catalogs.
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