1240MHz/1280MHz DIY Helix Antenna

Just finished with my helix antenna, designed for 1240MHz.
Used the helix calculator from http://jcoppens.com/ant/helix/calc.en.php.
Parameters were 1240MHz, 8 turns, 0.23 wavelengths.

My internal helix diameter is roughly 80mm with a coil spacing of 55.5mm. Wire diameter is 3.5mm/10mm².
Reflector size is 250mmx250mm. Center support is made from 6mm Balsa, soaked with epoxy resin.
For matching the IBCrazy wave trap method is used. Trap size is 30mmx15mm. See also http://www.rcgroups.com/forums/showthread.php?t=1377791

 


1.2GHz FPV Receiver customized control

In a previous post I described how to control the internal tuner module of a common 1.2GHz FPV receiver by a PC.

Since this setup was only for general tests I put some more efforts and put an AVR tiny44 in control.

On a 4 channel receiver I removed the original micro-controller and replace it by a AVR tiny44, running on internal 8MHz oscillator.
The tiny44 takes inputs from the 4 DIP switches and controls the tuner by I2C and the LED.

Attached to this post is the source code for the tiny44, written for WinAVR compiler.

With this code you have the following functionality:

DIP1 DIP2 DIP3 DIP4 Function
ON OFF OFF OFF Favorite frequency 1, default 1240MHz
OFF ON OFF OFF Favorite frequency 2, default 1280MHz
ON ON OFF OFF Favorite frequency 3, default 1276MHz
OFF OFF ON short OFF Manual tune, 1MHz up
OFF OFF OFF ON short Manual tune, 1MHz down
OFF OFF ON long OFF Auto tune, 1MHz up every 100ms
OFF OFF OFF ON long Auto tune, 1MHz down every 100ms

For DIP3 and DIP4 a long press will be >1 second.

The frequency set using DIP3 and DIP4 will be stored in EEPROM 5 seconds after last tuning action (DIP switch used).
After power cycle this frequency will be the start for new tuning.

Frequency range for manual/auto tuning is 850MHz to 2200MHz in 1MHz steps.

Wiring is given in the source code, header of main.c.

FPV Receiver Control
FPV Receiver Control
FPV_Receiver_Control.zip
Version: 1.0
13.8 KB
671 Downloads
Details

Skew-Planar Wheel Antenna 1240MHz

Here is my construction of a right-hand circular polarized, omnidirectional Skew-Planar Wheel antenna designed for 1240MHz.

Designed for ground station use, the antenna was made after the following tutorial from IBCrazy:
http://www.rcgroups.com/forums/showthread.php?t=1352583

0.75mm² copper wire was used for the antenna elements, some hot clue to fix everything and a piece of PVC tube to keep the antenna away from any metal parts of the tripod.

Due to fragility of the copper wire a Styrofoam ball will be used as a protection radom.

62mm is used as quarter wavelength.

Angle (green one) between the “eyes” on one S-shaped wire is 78-80°.
The angle (blue one) of quarter wave sections is 100°.

About fine tuning I found the following:
Green angle >90° will lower the resonance frequency while angles <90° will result in a higher resonance frequency.

Measuring results after proper tuning for 1240MHz:

Return Loss & VSWR


1.3 GHz Wireless AV set – Summery

Here is a little summery about the following 1.3 GHz Wireless AV set:

1.3 GHz Wireless AV Set

I got mine from eBay http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=300405272136

The set was shipped with the following items shown on the next picture:

1.3GHz Wireless AV Set

Transmitter

Technical details:

  • Supply voltage 12V
  • Supply current 280mA
  • Output power >28.5dBm
  • Size 65x50x20mm
  • Weight (measured) 62g complete; bare PCB 10g
  • SMA antenna connector
  • 3.5mm 4 pin AV+power jack
  • Gain adjustable
  • One button control
  • 15 Channels

Externals:

Transmitter FrontTransmitter TopTransmitter Bottom

Internals:

Transmitter PCB TopTransmitter PCB Bottom

As you can see there is no extra shielding around the PCB. Heat dissipation is mainly done through the bottom side of the PCB.
Some details about the components:

  • Philips TSA5520 1.3 GHz universal bus-controlled TV synthesizer
  • Futjitsu FLU10 RF power amplifier
  • 24C02B EEPROM
  • uC marked JX903D ???

Interesting is the separate EEPROM. Obviously the channel and frequency configuration is stored inside. Seems that is the first transmitter with possibility to use customized frequencies. I need to find out 😉

The EEPROM is used only to store the selected channel. The channel will be read on power up and stored on new selection.

Channel Frequency [MHz] Power [dBm]
1 993 28.3
2 1020 28.4
3 1050 28.3
4 1060 28.2
5 1080 28.2
6 1100 28.0
7 1120 28.0
8 1140 28.2
9 1160 28.6
A 1180 28.7
B 1200 28.6
C 1220 28.3
D 1240 27.9
E 1256 27.6
F 1280 27.3

Harmonic Distortion:

1. 28.04 dBm @ 992.97 MHz
2. -13.51 dBc @ 1.99 GHz
3. -16.15 dBc @ 2.98 GHz

Receiver

The receiver is similar to many others coming with such wireless AV sets. You have two buttons to control the receiver, one to cycle channels and a second to store channels for auto cycle.

1.3 GHz Wireless AV Receiver

A very interesting fact: The receiver is rated for the frequency range 0.9 – 1.56 GHz, split into 28 channels!

Channel Frequency [MHz]
1 921
2 993
3 1022
4 1051
5 1062
6 1081
7 1100
8 1120
9 1140
10 1160
11 1180
12 1200
13 1220
14 1240
15 1256
16 1280
17 1300
18 1320
19 1340
20 1360
21 1380
22 1400
23 1420
24 1440
25 1460
26 1480
27 1500
28 1520

Internals:

1.3GHz Wireless AV Receiver

The quality of the PCB looks much better on this receiver than on others I have seen so far. A large spring washer keeps the tuner module in place 🙄 But it’s also the first one with separate video amplifier. By default the gain of the video amplifier is fixed. But there are pads to solder a potentiometer (2KOhm) to adjust video signal level. Very useful if you drive more than one video input in parallel from the receiver.
Some details about the components:

  • TA7176 IF Amplifier+FM  Detector
  • EEPROM 24C02B
  • Philips NE592D Video amplifier
  • uC without marking
  • 78M05 + 78M09 Linear voltage regulators

The tuner module is common standard but the RSSI output pin is missing. You need to solder one yourself, the PCB is already prepared. See picture below.

Level [dBm] 	RSSI [V]
-115		0,786
-110		0,784
-105		0,781
-100		0,771
-95		0,758
-90		0,789
-85		0,938
-83,5		1,000
-80		1,160
-75		1,382
-70		1,601
-65		1,810
-60		3,370
-55		3,570
-50		3,660
-45		3,710
-40		3,750
-35		3,790
-30		3,870
-29		3,940
-25		4,340
> -20		4,340

The tuner uses the common TA1322FN Down converter in combination with a TA8804F FM demodulator. A 27MHz SAW filter ECS-D479.5B is used in the tuner. (Should be replaced by a ECS-D480A with 17MHz)

Antennas

No idea here. These antennas are usually crap so I will not test them for VSWR. Better stick with an Yagi and the Inverted-V.

Conclusion

With some rework this wireless AV set has a big potential for hacking and customizing.

Due to the fact that frequency configuration seems to be stored in EEPROM on both transmitter and receiver one can use almost any frequency within the RF specs of the devices. (For example not only one legal frequency at 1280MHz).

A separate shielding box needs to be made for the receiver. 65g for the original one is a way to much to carry up in the air. Also heat dissipation can be improved this way.

All in all I think that’s a good set, good hardware potential and better quality than I have seen before.

Channel RX Frequency
1 921
2 993
3 1022
4 1051
5 1062
6 1081
7 1100
8 1120
9 1140
10 1160
11 1180
12 1200
13 1220
14 1240
15 1256
16 1280
17 1300
18 1320
19 1340
20 1360
21 1380
22 1400
23 1420
24 1440
25 1460
26 1480
27 1500
28 1520
Auto 1540