ATV Rabbits - A 900 mhz ATV Experiment
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A group of amateurs in the Zebulon/Wendell/Youngsville NC area have been experimenting with converting 900 mhz rabbits for amateur ATV use by adding chassis mount BNC RF connections, tweeking and prodding, exploring their capabilities, etc. I'm currently "tinkering" with adding a power amplifier to bump the power up to a few watts. Here is an account of our activities so far. Pictures of this project will appear a little later.
Many VCR extenders, or "Rabbits" as they're often called, are basically wireless video senders and operate within the 902-928 mhz amateur band in DSB AM (double-sideband carrier injected amplitude modulation). This means that they operate much like a regular broadcast television transmitter... they transmit video in AM, and audio in FM on 2 subcarriers that are 4.5 mhz above and below the video carrier (broadcast transmitters transmit in VSB...only one audio subcarrier).
Various manufacturers produce these devices for low power part 15 unlicensed use. Consumers use them to retransmit video from a VCR or TV in one part of the home to another...wireless. They are inexpensive, and a curiosity to ATVer's (like us!). Hmmm... they come in pairs (a receiver and transmitter), and are already in the amateur 33cm band. A little more power, a decent antenna, and maybe a rx preamp and you'll have yourself a basic 33cm ATV station!
So far, We've been experimenting with the
Popular Mechanic's brand marketed by Walmart, and the Gemini "Rabbit"s.
Here's what we've found:
33 cm ATV Transmitters
The measured output of the Gemini rabbit on my
HP431 microwave power meter is about 8mw (about the same for the
Popular Mechanic's brand marketed by Wallmart). However, they are a bit
"dirty", with spurs popping up all around their output frequencies. A bit
of tweaking by spreading/compressing the inductors in the RF stage cleaned
them up nicely, and we've settled for a "clean" output level of about 0dbm
The Gemini's voltage
regulation is much better, and is more immune to changes in line voltage than the Walmart unit. The Walmart unit uses the same 12 vdc voltage regulator as the Gemini, but line voltage changes would cause Walmart unit to jump around by 300khz or more. Line voltage changes didn't seem to affect the Gemini at all. The Walmart transmitter, however, was much more stable from a cold start with less overall drift during transmit.
The Gemini "Rabbit" unit has rear apron controls
for video and audio level, whereas the Walmart unit did not. They could be
added to the Walmart unit, since the case is fairly large and not densely
The 4.5 mhz audio subcarriers are stable in both
units. The audio subcarrier is about 10 to 15 db down from the video
carrier in the Gemini, and down about 25db in the Walmart transmitter. Weak signal detection and recovery of audio is better with the Gemini's due to the additional energy in the audio subcarrier. Audio deviation is about +/- 25 khz, so all is OK there. The audio subcarrier frequency is independantly adjustable from the video carrier frequency in both models via the adjustment of a small ferrite core transformer.
Both transmitters also have an adjustment to affect the insertion level of the audio subcarrier. The Gemini's adjustability is from about -30db to about -10db. The Walmart unit is adjustable from _NO_ subcarrier to about -25db down. On thIt seems, from experimentation, that conventional TV broadcast receivers prefer the audio subcarrier to be down about 10-15db for good recovery of audio. Walmart unit, when adjusted for no audio subcarrier and transmitting black and white video, the power density of energy above -40db of the video carrier is contained in less than 1 mhz.
There are a couple of toroids in the in the
frequency determining stages of the vco in the Gemini "Rabbit" that have
to be secured with wax. We noticed that they would jump frequency a bit
when tapped or shook.
We wanted to bring one of the TX channel frequency controls out to the front panel via 10 turn pot for external "vfo" control, but with the cost of a 10 turn pot and vernier dial being $40ish total, we decided to let well enough alone in this budget project. After all, the RX is tunable!
33 cm ATV Receivers
Both receivers are pretty straightforward. They are simple tunable downconverters from 902-928+ down to broadcast TV channel 3 or 4. We found that both manufactures of downconverters will tune a little beyond the edges of the 33cm amateur band.
They are both pretty stable, even from cold start. The Gemini unit, however, seems to be a bit more forgiving of transmitter drift and is a little easier to tune to the TX frequency.
The Gemini unit is more sensitive, being able to detect video signals that are about 12dbm weaker than those detectable with the Walmart unit.
The Gemini receiver comes with the main tuning control on the front panel, whereas the Walmart receiver has it's main tuning control on the rear apron.
We discovered that the Gemini receivers, while having a tunable rx bandwidth of 35 mhz or so, don't always tune from the bottom of the 33cm band. One of the units we modified started tuning at about 913mhz. This is easily corrected.
The rear apron mounted whip antennas on the transmitters were removed, the paint was filed/sanded away for good contact with a connectior, and chassis mount BNC connectors were installed. This makes connection to an amplifier or "real" antenna simple.
The 3 front panel selectable TX frequencies of the Gemini are controlled by the settings of 3 internal pots that control the VCO. The same goes for the 2 rear panel selectable frequencies of the Walmart unit as well. We adjusted one of the settings to 910.25, our ATV repeater's AM input, and the other two to 917.25 and 923.25, respectively. On the Walmart unit, we simply skipped 917.25. Stability after 5 minutes of warmup is about +/- 200 khz on the Gemini, and about +/- 100khz on the Walmart tx...not too bad. They have no problem staying within the passband of the supplied receivers. Holes were drilled in the cases to allow easy access to the frequency determining pots from outside.
Holes were also drilled in the cases of the transmitters to allow access to the little silver adjustable transformer that controls the operating frequency of the audio subcarrier. This transformer was adjusted for +/- 4.5 mhz from the video carrier.
Transmitter purity and power adjustments were made with the aid of a spectrum analyzer and microwave power meter. Coils in the RF section were spread/compressed to attain minimum spurs and maintain an output of 0dbM (1 mw). This is adequate to drive a number of inexpensive linear devices currently on the market for 800-1000 mhz commercial users.
The receiver antenna modifications were performed as in the transmitters, providing BNC connections on the rear panel.
The receiver tuning range of the Gemini units is determined by a calibration resistor located immediately behind the receiver main tuning control. It's the resistor that looks to be "tack soldered" in place after the unit it assembled. Increasing resistance lowers the frequency of the tuning range. This seems to be affected by other circuit variables, so no set value of resistance is recommended. Experiment! We simply set the transmitter to 910.25, and substituted a value that allowed ease of tuning near the bottom end of the main tuning control's range.
The Gemini receiver required no additional modifications, but the Walmart unit was modified to relocate the main tuning control to the front apron for ease of operation.
The next step in this project is fabrication of a simple amplifier to get the output up to about 1 watt. We're currently experimenting with a number of simple and inexpensive devices to get the output up to 1 watt or so. I'll report back on this soon.
The TX's are DSB, so I guess I'll cross the VSB bridge a little later. While DSB may be a problem on 70 cm, 33cm is "no man's land" in this part of the world.
Sources for Rabbits...
So far, this is were the problem arises. It seems that Walmart has dropped the Popular Mechanics brand of VCR Multiplier from their line. There are still some out there floating around in inventory. Does anyone else know who distributes this brand??
The Gemini products are handled by a number of distributors: Home Depot, Roses, Western Auto, etc. Most distributors are also dropping these from their product lines due to lack of sales. Check with your local dealer... I got a pretty good discount on mine due to being dropped from stock.
Analog and Digital RF Modulators from Techmall
Supercircuits Supplier of wireless video products
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