This is a simple 2 positions antenna switch that I made to compare receiving antennas; a tool that I recommend to everyone that like to play with antennas without having to walk back and forward to switch cables.
A remote controlled switch allow to see how the signal changes in an instant and unless you tune on a continuos signal it will be difficult to make a comparative test without it.
The relay is a DPDT (double pole double throw) relay HK19F and we switch both center conductor and shield. Although for transmitting antennas it is recommended to switch only the center conductor and keep the shields connected together I prefer this less intrusive solution for receiving antennas.
Connections between the relay and the connectors:
I used coax of good quality but not the correct cable for this job because it is very hard and makes difficult to prepare short pieces, infact I had to remove the outer jacket. In any case, considering the very short length i don’t think that this makes any difference.
The correct cables to use are probably RG174 for 50Ω and RG179 for 75Ω ; if you know more flexible ones please let me know.
The relay’s coil is 12 Vdc, controlled by an inexpensive RF remote control, the ones used for single color led strips.
Initially I wanted to insert the RC receiver inside the box but I was afraid about the noise produced from the circuit so I preferred to keep the box smaller and place it in line with the DC power supply wires outside the box.
A 12Vdc power supply line is needed at the antenna mast. I have my antennas on the balcony so I brought the RG6 cable and two wires for the power supply that can also be used for the LNA. Alternatively you can implement a bias tee circuit and use the antenna coax for all; I am not a big fan of the bias tee.
Another solution for non permanent installations is to use a battery pack.
The same relays are available with 5Vdc coil if you prefere but the controller works at the bottom limit (rated 5-24 v).
Coil circuit: the 12Vdc from the RC receiver go direct to the relay coil which has in parallel:
- a flyback diode to short voltage spikes when switching off, any diode is good, i used 1N4007
- a capacitor to short the noise produced from the rc receiver; oh don’t ask me the value, i just picked a bypass capacitor from an old circuit board. I am not even sure if you need it but a capacitor between DC wires never hurts
- a led with a 560Ω current limiter resistor in series to visually see the relay switching
I chose to use BNC connectors on my RG6 cables, this also allow fast switching of the cables when I install and remove antennas because my antennas are in the balcony in a non permanent installation and i use PPE conduits and pressure clamps for fast installation. I must be able to remove and install antennas in less than one minute.
Now that you had the description of the antenna switch here is the extra bonus:
Periodic switching function to compare antennas performance in presence of strong fading, as described in the LZ1AQ article
I did not test it with Linrad like Chavdar suggested but i can make it switch and adjust the time and interval.
This antenna switch should be used for HF; it can be used for VHF or even UHF but you have higher insertion loss.
I checked with Welle.io for DAB+ on 223-227 MHz and i did not find any difference in the SNR.
I used SDR# to find some continuous signals and checked the Signal Peek, Noise Floor and SNR with and without the switch inserted (bypassed using BNC F/F adapter); however the values displayed are not stable so i took 7 screenshots and calculated the average.
Here are the results:
161 MHz insertion loss 0.2 dBFS
447 MHz insertion loss 2.0 dBFS
However consider that some little loss on higher frequencies will not change the functionality of the switch as it is used to test and compare two antennas.
Extending the remote control range
the distance range is more than 10 meters at direct sight but it reduces if you move around a corner, however there are a couple of ways to extend it according to your installation:
My pc desk is around a corner and the remote control will not work all the times but i have found that if i keep it closed to the power supply wires it works because the wires receive the RF and bring the signal to the receiver outside. I can do this because my DC power line is connected to a power supply closed to my pc desk.
I made a little box with the DC wires inside and the remote control laying on the top; i have connected the 12v positive to a piece of adhesive copper foil sticked on the top of the box under the remote control.
I can just lay the remote control on the top of the box and use it.
In a first time i tried wrapping a coil aroud the positive wire but it was not as reliable as the direct connection.
My DC power line is 15 meters long; i cannot say if with this system you can send the RF signal up to the roof but you can try and let us know.
If the DC power supply line is not closed to the pc desk you can try to add an external antenna to the remote control.
Here below are the pictures of the circuit board:
Q. From my pc desk i cannot see the LED on the antenna switch so i loose count of which antenna is selected
A. Spend $2 more and buy a second remote & receiver; keep the remote for spare and pair the two receivers; keep the second receiver on your desk connected to a LED
The receiver can be paired to the remote control pressing keys 100% and 25% together within 5 sec. after powering on the receiver.
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