Ribamodel Team

Portuguese Radio Control Airplanes, RPV/FPV/UAV


Santiago 1200 CB Antenna Hack - 35Mhz = 5km range

In the early beginnings of my research on FPV, and well before the UHF TX/RX, I was researching for a way to have more range with my 35Mhz Radio (JRX3810). After some research, and some conversations made with a friend of mine who is a Amateur Radio addict, we tunned a CB antenna which gave me a range of 5km. Here I will explain how I did it. This antenna can also be used with my 13W 35Mhz amp.

1 => The antenna

The antenna that I used was a Santiago 1200 CB Antenna.

Type: 5/8 λ
Frequency: 26-28 MHz
Bandwidth: 2000 KHz
Impedance: 50 Ohm
S.W.R.: < 1:1.2
Max. Power: 1200 W RMS, 2400 W P.E.P.
Height: 1950 mm
Mounting hole: Ø 16 mm
Cable: RG-58 4.5 m
Relative Gain: 5 dB
Base type: SO-239

1.1 => The antenna, where to buy

Here's some links where you can buy this antenna:
    http://www.shopmania.co.uk/shopping~online-audio-video-accessories~buy-santiago-1200-1-95m-1200w- steel-whip-cb-antenna~p-8390517.html

2 => Hacking the antenna

 2.1 - First thing you will need to do, is to take out the cap that is protecting the coil of the antena.
Desoldering the antena coil

2.2 - After that, you will need to make a bypass with a 16 to 22 AWG electric cable for a determined position of the coil to the connection of the antenna as shown below. To know the exact place of where to make this bypass, you will need a SWR meter from amateur radio. The reason you will need it is because you need to tune the antenna to exactly match your frequency, so that you can have the best output power on the antenna. I tuned mine for 35.170Mhz.
Making the bypass

2.3 - Protect the unused coil from the bypass with some electric tape as shown below so that they don't enter in contact.
Protecting unused coil with electric tape

2.4 - Cut EXACTLY HALF of the cable from the magnetic base of this antenna. The reason is because as more cable you will have, as more DBi you will loose along it's way.

2.5 - Put the cap back on, and check once more the tuning of the antenna with a SWR meter. If everything is alright, then you are ready to go.
Finished antenna

3 - FAQ About this project

[Q]: Why do I need to make a bypass in the coil instead of cutting the length of the antenna until I have it tuned into my frequency?
It's not wise to cut the length of the antenna, because as more length the antenna has, has greater will be the irradiation of radio signal.

[Q]: Can I use this method in another type of antenna?
Yes, just follow the same methods as described before.

[Q]: Why do I need to cut the cable from the magnetic base exactly in half?
As longer the cable is, and more DBi you will loose along the way. Also, I had better results when I cutted the cable exactly in half than having it even smaller or a little bigger.

[Q]: Can I leave the cable length as it is by default without having to cut it?
You can, but I seriously advise you NOT do to it, because you wont have the same power output in the end.

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