Aerial Switch box details

FM antenna discussion
Post Reply
daveB
Posts: 1002
Joined: 07 Oct 2014 16:49

Aerial Switch box details

Post by daveB » 02 Apr 2017 23:15

[I'm bored - no DX - solar light qrn...]

So let me share a fairly cheap and easy method of cobbling together an aerial patch box which provides a much higher isolation than can be achieved with reasonably priced relays. Using a 110 mm x 58 m x 28 mm diecast box you can fit up to six antenna input sockets plus a seventh for the output.

As the pictures below show the construction is straightforward - spacing between sockets along the side is 20 mm. Then it is simply a case of using a very short piece of wire around 22 swg to link the phono (or jack socket to the appropriate f-connector. Then a jumper lead goes between the output scket and whichever input you want. I've yet to test whether some thin flexible coax would be better than the screened audio cable but I doubt if I could tell the difference

box with 3.5 mm mono jacks https://app.box.com/s/way57i250wtdd891wp126onzc0qz3z8i

box with phono sockets https://app.box.com/s/p4s71qqckdtr9ipx63ldt1dvvhkzjpnd

the one with the 3.5 mm mono jack have a built in switch - this is used to put a 68 ohm (or 81 ohm) resistor across the aerial socket when it is not selected. I did this to try and stop interaction between the different feeds as most of the inputs to each box come from aerial splitters.

You can see for yourself the degree of isolation in the screen-shots below - this is the worst case signal from Heart where the isolation is around 55 dB. This compares to around 30 dB in the FM band for a cheap "rf" relay.

In contrast the High Isolation Cable switch that you can get from Maplins claims 80 dB, and certainly there is no trace of a 60dB signal when the switch selects the input with no aerial - but that's all it does - 2 inputs.

switchbox 3.5mm jack connected https://app.box.com/s/c1fgvz6asbsqxdm01gota4aid70ip6sn

switchbox 3.5 mm mono dis-connected https://app.box.com/s/mmzguoegzazuw1neozdnalsi9mmf1euf

switchbox phono connected https://app.box.com/s/rn3c21dnoj37rqcv4dhkkjgseg2tsq6z

switchbox phono - disconnected https://app.box.com/s/cxncx53ee32qutd80exbj84bhd7lsmu6
SDRPlay, AirSpyMini, RTL dongles, SB920s, Yamaha TX930. 5 ele compact yagi on rotator at 18ft agl. FM5 facing east, FM5 facing south, FM3 facing NE, OIRT dipole, 3-ele Moxon vertical facing east.

simbeav
Posts: 361
Joined: 14 Oct 2014 15:00

Re: Aerial Switch box details

Post by simbeav » 03 Apr 2017 14:57

Thanks for posting Dave. Would make things easier than my constant screwing and unscrewing (and in th process loosening the connecton.

What is the part number of the peg ?

daveB
Posts: 1002
Joined: 07 Oct 2014 16:49

Re: Aerial Switch box details

Post by daveB » 03 Apr 2017 16:27

What is the part number of the peg ?
I knew someone would find the pictures amusing :D

You can't see that the switch boxes are sat on top of a Sony SB920 tuner and are held in place by double sided tape. With all the cables coming out of the back of the box it needs a support to keep it stable - anything suitable will do - provided it's cheap!

I've had the phono switch box for at least a couple of years and have only had to replace a couple of them due to excess wear - which is not bad going. I also need to get another couple of right angle F-male to f-female adapters and finish off the other box with a sixth input. As I've got fixed aerials in various directions it is much quicker to un-plug/plug between them to get the best direction, than to turn the rotator. That gets done when I've decided what direction is best.

In the picture of the box with the mono 3.5 mm jacks you can see a pair of ferrite rings round an f-male to male connector. Just in sight is the Airspy mini which is connected direct to the box - it is hard to see as it is kitted out with a set of black heatsinks so I can run it at 10MBPS for wide band recording - without it getting too hot.
SDRPlay, AirSpyMini, RTL dongles, SB920s, Yamaha TX930. 5 ele compact yagi on rotator at 18ft agl. FM5 facing east, FM5 facing south, FM3 facing NE, OIRT dipole, 3-ele Moxon vertical facing east.

Post Reply

Return to “Antennas”