Edited 07/16/2016

Standardized Test Setup for Antennas

DISCUSSION and BACKGROUND

We had a set of active antennas in operation at Kalaeloa for over a year, but the data has never been analyzed. The signal level is down about 10dB from the regular base-loaded antennas. For now, that is acceptable since we have 10dB attenuators on the regular antennas, but when we get the TX quad properly nulled (and in future installations) we will need to roughly match the old antennas.

Since we hope to make active antennas a major feature of all future systems, it is critically important to properly characterize and understand the properties of these antennas at this time, rather than just continue guessing.

Hilo antennas are 1/16 (27" ?)
Kalaeloa antennas are 1/40 (18")

Fig. 1.1. Older Kalaeloa active antennas: ~-10dB compared to passive antennas

Fig. 1.2. Current Model 2 Active Antennas. as deployed in Hilo.

For Mexico, we will use SXLP-23 Low Pass filters for 12.5 and 16MHz to reduce the noise bandwidth.

For 6 MHz operation, the SXLP-10.7 will further reduce noise bandwidth for maximum range.

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PCB TEST PROCEDURE

Once the antennas are assembled in the PVC pipe, it is very difficult to service them.

Given the number of antennas to be assembled, this could be a major waste of time.

A quick and repeatable test of the bare PCBs is essential

This test must check the normalized gains to catch all errors

Fig. 2.1 Testing the bare Active Antenna PCB before assembly

Fig. 2.2 Testing Setup

This test procedure uses a 15pF series capacitor to simulate the impedance of a shortened antenna.

Set up MCI RF Power Meter with the Bias-T
There MUST be a 100 ohm resistor in series with the 12 volt supply to PREVENT BURN OUT.
The attenuator is set to provide roughly 0dBm output on a typical Active Antenna PCB.
- The test data will then be in +/- dB deviation.
16.7dB seems to be a proper value for the attenuator. I will solder a permanent two-resistor attenuator to the antenna simulator.
- A 56 ohm resistor and a 10 ohm resistor should be about right

KEEP THE BIAS-T, ANTENNA SIMULATOR AND CABLES IN A PERMANENT KIT FOR UNIFORM TESTING

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PVC ASSEMBLY PROCEDURE

Fig. 3.1 Assembling the Active Antenna.

Assembly procedure: (subject to revision)

Once completely assembled and sealed, the PCB is essentially "disposable".
The solid #12 AWG electric wire should first be soldered to the PCB
Place a bead of polyurethane caulking around the Type-N connector, inside and out
- We need to find the best alternative to RTV... It out-gasses acetic acid.
Tighten the connector nut
Let cure overnight so that the seal does not get broken by manipulation.
PVC parts are "snaked" onto the wire one at a time.
PVC joints are sealed with PVC cement.
Finally, drill a hole in a PVC cap for the antenna wire
Seal the wire exit thoroughly inside and out
Seal the cap with PVC cement

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SPICE simulations of Active Antenna Circuits:

Fig. 4.1. Frequency response (SPICE simulation): The 3MHz high-pass at 6MHz
The low-pass corner will move down from 40MHz to 23MHz.
For operation at 6MHz, we may be able to use SXLP-10.7 filters.

We need to do a VNA test of the boards with SXLP-23 and SXLP-10.7 filters

The series capacitor in the simulator should be adjusted for frequency to provide -j420ohms.

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TEST PROCEDURE: What is missing is to normalize the gain

(compared to a standard passive antenna.)

Fig. 5.1. Test Setup triangle for active and passive antennas. I recommend driving a broadband ground rod at each base rather than fiddling with radials.

Note: This is scaled for 27.3MHz. Half-wave spacing = 5.5 m. (18 feet.) and Quarter-wave vertical element 100 inches. (8.4 feet) seems much more reasonable than 16MHz.

The setup consists of three construction buckets with holes in the lids to use as antenna bases. Three lengths of half-wave nylon cord are used to uniformly space the triangle.

This setup will allow us to quickly pre-fabricate and test antennas before going in the field. The mini-element of the active antenna will be trimmed incrementally during testing to obtain a full range of data points to characterize the performance of the active antenna and to match the gain of the regular antennas.

The reference quarter-wave antenna will be used for the TX port of the VNA2680. With this setup we will immediately get the full frequency / phase response of the antenna pair.

Antennas display reciprocity: that is, they behave the same on transmitting and receiving. This means that an identical pair of antennas will have a response which is the square of the individual response. This also means that dB response of an unknown antenna can be obtained by subtracting the response of the known antenna. The relative response of two antennas (e.g. active and passive) can be deduced by comparing them directly in an equilateral triangle configuration.

Since the active antennas should be quite flat in their response over the bandwidth of a passive antenna, we should get a good response profile for the reference quarter-wave TX antenna so that we can then get a good estimate of the individual antenna responses. This can also be done by taking the ratio of the one RX antenna to the broadband active antenna.

There is other questions here for which I presently have no answer. The Kalaeloa passive antennas are 1/5 wavelength. The present active antennas are only 1/40 wavelength. There are two factors which kill the sensitivity of a short antenna. One is the length of the "e-field" probe, which intuitively might be linear. But the equivalent "capture area" (the area of the watts-per-square-meter field that the antenna can actually capture) is probably steeper. The second problem is the source impedance (and thus the mismatch) of the short antenna. With impedances running in the hundreds of ohms (and almost purely reactive), not much of that power is transferred to the load. Yet at only 1/40 lambda, they are only 10dB down.

 FIELD TEST PROCEDURE

Clip 18 foot nylon cords between the bucket bases.
Drive a 3 foot ground rod 2 feet into the ground at each antenna base and ground the coax shield at that point.
Run the three 20 foot coax cables back to the test equipment.
Use the step attenuator to run the amplifier at a reasonable power level.
Use a fixed 10 dB attenuator to guarantee that the input to the amplifier stays below -3 dBm.
The first step is to get the basic measurements "around the circle".
The actual antenna gains will be measured at the peak of the resonance of the TX antenna.
The passive antenna must be tuned to that resonance. This is the "standard gain".

(If easier, we may tune the TX up in frequency by trimming the tip down.)

The Active Antenna should start at 1/4 wave and be trimmed down to 1/20 wave in small increments.

This may initially require another 20 dB attenuation of the TX signal.)

Select the Active Antenna length that matched the passive antenna gain and restore that wire length.
Use that data set to determine the bandwidth and shape of the TX antenna. (assuming the Active Antenna is relatively flat.)
Subtract that TX antenna gain curve from the passive antenna to get its own gain curve.

Saving Data

Save the bitmap screen shot.
Use a descriptive filename with incremental numbers:
Active55cm, etc. (vertical element length)
Passive03, etc.
Save the Port B Graph (data file) with the same description and filename.

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ActiveAntenna