Author Topic: What are some dos and donts to building an antenna switching box?  (Read 987 times)

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Offline raspberrypi

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I have an SDR under my bed that I like to listen to at night, I have three cables coming into the bed room from various locations in the house where the antenna are mounted. I'm kind of bed ridden most night so I want to make a box that can just switch between the long wire SW, the little ADSB dipole under the bed, and the 1m yagi. Best would be have the box mounted to a wall next to the bed. But what kind of switch can handle kHz to 2 Mhz with low noise or dropping db? I know it should be in a metal box. Would a splitter work where you lose 3db on each tap work? or would that create a huge antenna that would just kill the band width of the three antennas? Can you use just a regular three way rotor switch?
I'm legally blind so sometimes I ask obvious questions, but its because I can't see well.
 

Offline evb149

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Re: What are some dos and donts to building an antenna switching box?
« Reply #1 on: March 21, 2017, 02:42:47 PM »

Do not just combine them with a 4:1 or 2:1 splitters backwards.  I mean if you already HAVE the splitters and connectors and cables to try,  sure, try and see how it works for you.  But it is likely at best far from optimum.
 If you already have mechanical switches or relays  you can try those if you like but most will have poor matching and capacitive loading and maybe high inductance too.

Check out the RF direction finding thread on here, they switch between several antennas and it ranges from UHF to VHF and it works "ok".  There are probably different suggested designs, but most I think are using diode type switches.

If you want to get some proper PIN diodes they will switch better if used properly.  Maybe BAP50?  The "off" isolation can be a little weak, maybe 30 orrrrrrr 40dB I guess, check the data sheets and application notes.  For more than that you probably need miltiple stages.

Anyway the really "proper" and "easy" way is   to use RF switch ICs or mechanical switches that are intended for RF use up to the highest frequency you want to use.  But maybe that is too much of a project / expense for you.  There are probably amateur radio oriented products you can just buy for a 4:1 or whatever antenna switch with a dial knob but I don't have one to suggest.

If you want to experiment with a diode switch and not buy parts or use even more common ones you might be abllle to make something adequate with 1N914  or 1N4148 common low capacitance switching diodes.  I think the reverse capacitance is still around 2-6pF range so not the greatest but probably good enough?  Maybe the rf direction finding thread has such examples?


I have an SDR under my bed that I like to listen to at night, I have three cables coming into the bed room from various locations in the house where the antenna are mounted. I'm kind of bed ridden most night so I want to make a box that can just switch between the long wire SW, the little ADSB dipole under the bed, and the 1m yagi. Best would be have the box mounted to a wall next to the bed. But what kind of switch can handle kHz to 2 Mhz with low noise or dropping db? I know it should be in a metal box. Would a splitter work where you lose 3db on each tap work? or would that create a huge antenna that would just kill the band width of the three antennas? Can you use just a regular three way rotor switch?
 

Offline evb149

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Re: What are some dos and donts to building an antenna switching box?
« Reply #2 on: March 21, 2017, 03:08:08 PM »
There are somewhat inexpensive A/B switch boxes sold by places like Amazon etc.
http://www.solidsignal.com/pview.asp?p=tv-0151
http://www.ebay.com/bhp/coax-switch
https://www.amazon.com/Coaxial-A-B-Switch-1/dp/B0002ZPIQ4

I['m sure someeone must have inexpensive 4-way ones...
 

Offline vk3yedotcom

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Re: What are some dos and donts to building an antenna switching box?
« Reply #3 on: March 21, 2017, 09:34:51 PM »
For VLF through to low VHF, especially for receiving, an ordinary rotary switch would be fine. 

Put in a metal box with antenna sockets screwed to it and use switch to switch between the inner contacts. 

If you don't have a rotary switch you could use two slide switches hooked up in such a way that allows antenna A or a choice of antenna B or C.
NEW!  Getting back into Amateur Radio: What's new and what you forgot.  Quick restart guide in a 120 page Kindle ebook.  $US $4.99 or equiv.  Just search 'Getting back into Amateur Radio' on Amazon.
 

Offline Chris Wilson

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Re: What are some dos and donts to building an antenna switching box?
« Reply #4 on: March 22, 2017, 12:02:50 AM »
I have built a few switches, mechanical rotary and relay based for those low frequencies. Up to 2 MHz is pretty easy and either the commercial boxes with sockets on them, or just a rotary wafer switch with the needed positions should be fine. It's when you get higher in frequency or TX'ing big power through them that it gets a bit more involved. I assume 2MHz isn't a typo, it's with you mentioning a Yagi that I wonder if you also receive on VHF??
Best regards,

                 Chris Wilson.
 

Offline evb149

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Re: What are some dos and donts to building an antenna switching box?
« Reply #5 on: March 22, 2017, 05:25:23 AM »
Some of the stuff he mentioned was 1GHz UHF, I didn't pay close attention to the rest since I assumed the highest frequency would be more sensitive to parasitics and noise figure.
 

Offline raspberrypi

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Re: What are some dos and donts to building an antenna switching box?
« Reply #6 on: March 26, 2017, 01:32:09 PM »
I ment up to 2GHz, thats what the SDR can do...
I'm legally blind so sometimes I ask obvious questions, but its because I can't see well.
 

Offline HowardNamath

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Re: What are some dos and donts to building an antenna switching box?
« Reply #7 on: March 31, 2017, 02:26:49 AM »
These aren't exactly cheap, but here are a couple of Analog Devices RF switch eval boards that might fit what you're trying to do. You could potentially throw one in a project box with a power source and control switches:

This is a SP3T RF switch that uses a +5V supply (so you could maybe use a filtered USB supply to power it): http://www.mouser.com/ProductDetail/Analog-Devices-Inc/EV1HMC245AQS16/?qs=%2fha2pyFaduihYW547tXjKjpe0RLSci6Qd7qAjm8N21KTMbV%252bVgCTxA%3d%3d

This is a SP4T RF switch that's a bit cheaper (and lower-performing): http://www.mouser.com/ProductDetail/Analog-Devices/EVAL-ADG904EBZ/?qs=sGAEpiMZZMv9Q1JI0Mo%2ftcrEDU6JWfr1
 
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