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Amateur Radio Antennas - For discussion of all amateur band designed antennas and related accoutrements. This includes base, handheld, mobile and repeater usage. For commercial antennas on the amateur bands please use Commercial Radio Antennas below.

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Old 04-24-2017, 2:20 PM
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Default DIY Antenna Switch Project

I've been looking into this lately and I'm "almost" ready to start
an actual build. My basic plan is for an electronic switch box.

I have plenty of Omron (Ice Cube type) quad relays (12vdc)
My plan is to use the relays to break the shield as well as the
center conductor for each output. Each relay will have a pushbutton
switch and an LED. This box will NOT be used for multiple antennas,
but for 1 antenna and multiple radios.
The relays will be mounted on
a piece of DIN rail and isolated from the metal box and the bases
are plastic so there will be nothing common to all relays except for
the power to charge the relay. The SO239 connectors will have to
be mounted on a piece. of plexi or something non-conductive to keep
them isolated from each other or use shoulder washers.

The only thing Im not sure of is:
1)Will there be a coupling issue by using relays?
2)Will the relays introduce some RF?
3)Is there anything in my design that would create loss?
4)Can I use 14awg solid wire for connections or should I use short
pieces of coax to connect relays to the SO239's?

As always, I love this hobby and love having this forum
to vent, explore, and learn.
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Old 04-24-2017, 2:55 PM
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Quote:
Originally Posted by SpugEddy View Post
The only thing Im not sure of is:
1)Will there be a coupling issue by using relays?
Likely some, unless these are specifically coaxial cable relays. Coaxial switching relays are available, but they tend to be expensive. For receiving use, I doubt any coupling will be an issue.

Quote:
Originally Posted by SpugEddy View Post
2)Will the relays introduce some RF?
If you are driving them with DC, then no. If you are driving them with AC, then yes, but only at the frequency you are using (60 hertz).
However, when relays open and close, you may get a little "pop" on your radio.

Quote:
Originally Posted by SpugEddy View Post
3)Is there anything in my design that would create loss?
Everything is going to have some sort of loss. The best you can do it to either keep the antenna/radio leads short, or use coaxial cable. There will likely be some loss inside the relay (if you aren't using coax relays) from the impedance mismatch. For scanner use, I wouldn't worry about this.

Quote:
Originally Posted by SpugEddy View Post
4)Can I use 14awg solid wire for connections or should I use short
pieces of coax to connect relays to the SO239's?
You could, and it would probably work fine. Better choice would be to use small coax. RG-58 might work, but RG-174 might be easier to work with.

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Originally Posted by SpugEddy View Post
As always, I love this hobby and love having this forum
to vent, explore, and learn.
Not an uncommon project. I guy I used to work with built something similar, but he went with a raspberry PI controller. Too far, in my opinion, but that can be part of the fun.

Many, many years ago, when I was in my teens, I built a small antenna switch using a 8 position rotary switch from some old piece of equipment and used it for switching short wave antennas. I had one of the positions set up to ground the radio antenna input.
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Old 04-26-2017, 12:24 AM
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Here is the plan so far.
Anybody see any flaws, please point it out
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Old 04-26-2017, 1:00 AM
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Personally, I'd add a fuse in your power connection. But, as long as you have one somewhere close to your power source, you'll be OK.
Maybe indicator lights unless the switch positions are obvious.

There's ways to make it fancy and complicated, but I like the simplicity. It would be easy to over think/over engineer something like this. For receiver use, it's just fine.
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Old 04-26-2017, 7:57 AM
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If you mount the relays in a metal box, I don't think you need to switch the coax shields. As long as the outer shell (aka "shield") on all coax input and output connectors is bonded to the metal box, the box will serve as an overall shield.

One thing lacking from your design is an interlock between radios. The way you have it drawn, there's nothing stopping you from turning on multiple switches, thus activating multiple relays. Don't know if that's a problem in your situation or not.
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Old 04-26-2017, 7:58 AM
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+1 with mmckenna and W9BU posted above. Plus the chokes, if correct ones, are added benefit too.
What type of relays will you be using?
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Old 04-26-2017, 11:24 AM
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It looks like a nice, straight forward design, Eddy.. but I'm going to throw in a caveat---.
What frequency(s) are you planing on using this? If its HF, it should be fine for what you've planned. If, as I suspect, you wish to use it on VHF, and especially anything high'r, there is going to be SWR issues and insertion losses. That network is going to look like a big lump, dump'd smack into the middle of your feedline. At UHF the wires you use to connect the relays- everything in there,- will take on the configuration of some sort of resonant circuit(s.) The effects, and losses. may be minimal,- or horrific-- you won't know until you've constructed it.
.
To that end, make everything as 'UHF Friendly" as possible. I'd try to get direct connections to your coax connectors- no bridging with any cable length ** or bare wire. This will be a mechanical challenge, I know- but I cringe when I see patching cables in such RF arrangements for anything above 200Mhz. The distance between these connectors is significant- the shield must be direct, solid and part of the enclosure. This enclosure itself may form part of the matching circuit. Bye the bye, I use silver plate brass for such designs (exotic, I know, but it can make quite a difference depending on the frequency)
.
The use of mechanical RF relaying is fraught with peril- if you could see what this might look like on a spectrum line analysi'zr you probably think twice about trying your hand at this-- but do it anyway!.... Its a great project.
.
Good luck
.
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Last edited by Coyote-Frostbyte; 04-26-2017 at 1:06 PM..
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Old 04-26-2017, 1:39 PM
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Quote:
Originally Posted by mmckenna View Post
Personally, I'd add a fuse in your power connection. But, as long as you have one somewhere close to your power source, you'll be OK.
Maybe indicator lights unless the switch positions are obvious.

There's ways to make it fancy and complicated, but I like the simplicity. It would be easy to over think/over engineer something like this. For receiver use, it's just fine.
There will be an inline fuse that I simply forgot to add into the plan.
The switches are, in fact, LED switches. I was going to have separate
LED's but decided to go with LED's embedded into the switches.

Quote:
Originally Posted by W9BU
If you mount the relays in a metal box, I don't think you need to switch the coax shields. As long as the outer shell (aka "shield") on all coax input and output connectors is bonded to the metal box, the box will serve as an overall shield.
The box was going to be metal, but I found an old unused plastic box that will be
just right for this project. I wanted to eliminate the chances of the "box" itself from
becoming any part of the circuits.

Quote:
Originally Posted by W9BU
One thing lacking from your design is an interlock between radios. The way you have it drawn, there's nothing stopping you from turning on multiple switches, thus activating multiple relays. Don't know if that's a problem in your situation or not.
Very nice catch. I've given this more thought than the rest of the project. The only
way to do this is to create some sort of ladder logic. Meaning a 2nd relay for each
switch. (ie. Switch 1 is on. Relay 1 is on. Relay 1-1 turns on and places an "open"
across the Positive power of the other 3 relays.) I don't have space for 8 relays so
I'll have to rely on the lighted switches.

Quote:
Originally Posted by KC4RAF
What type of relays will you be using?
Omron 12vdc MY4. Quad pole Ice Cube type bifurcated relays.

Quote:
Originally Posted by Coyote-Frostbyte
What frequency(s) are you planing on using this?
10 and 11 meters only.

Quote:
Originally Posted by Coyote-Frostbyte
I cringe when I see patching cables in such RF arrangements for anything above 200Mhz.
The RG174 patch cables will be approx. 3-4 inches, soldered to the SO239, and
screw terminated to the relays.

I thought about the cataclysmic fusion and collisions of RF in this
project a lot, and I came up with the following conclusion:
I've seen the inside of several "antenna switchers" such as Workman,
Optek, Para Dynamics and they're nothing more than a switch. They
like to use common connections (ie, the shield side of the connectors)
my design is making and breaking ALL connections to avoid all common
connections and the obvious backfeed of any signal. (Not that there won't
be "any" RF leakage)

Last edited by SpugEddy; 04-26-2017 at 1:49 PM..
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Old 04-26-2017, 1:46 PM
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here's a couple pictures of what I have , so far.
This is just the relays assembly. I'm waiting for the
rest of the SO239 connectors, the RG174 coax, and the
switches.
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Old 04-26-2017, 2:32 PM
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Ahhh !.. 10/11 metre's.... the design get's Coyote's stamp of approval--it should be great.
.
(Sorry, but I see everything as a microwave.... )
.
..................CF
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Old 04-26-2017, 3:07 PM
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Quote:
Originally Posted by Coyote-Frostbyte View Post
Ahhh !.. 10/11 metre's.... the design get's Coyote's stamp of approval--it should be great.
.
(Sorry, but I see everything as a microwave.... )
.
..................CF
So do I Coyote. That's why I posted here. Some of my best ideas
were perfect examples of what NOT to do.
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Old 04-26-2017, 3:18 PM
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Also, here are the ferrites I'm looking at
I'm trying to find the non-snap on but not having
much luck with 3mm

stardrift 10-Pack 3mm Inner Diameter Ferrite Core Cord Ring RFI EMI Noise Filter
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Old 04-26-2017, 4:44 PM
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Quote:
Originally Posted by SpugEddy View Post
Very nice catch. I've given this more thought than the rest of the project. The only
way to do this is to create some sort of ladder logic. Meaning a 2nd relay for each
switch. (ie. Switch 1 is on. Relay 1 is on. Relay 1-1 turns on and places an "open"
across the Positive power of the other 3 relays.) I don't have space for 8 relays so
I'll have to rely on the lighted switches.
I was under the mistaken assumption you wanted to be able to switch all radios in to the antenna.

In that case, you could simplify by using a rotary switch. If you get a multi level version, you could switch the antennas with one, and LED's with the other.

But, not as much of a challenge, and that would take some of the fun out of it.

If there are extra contacts on the relay that won't be used, you could set it up to lock out the other relays.
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Old 04-26-2017, 6:00 PM
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Quote:
Originally Posted by SpugEddy View Post
Here is the plan so far.
Anybody see any flaws, please point it out
OK I am very confused...

1) Why do you have ferrite beads on the center conductors? These will present a high VSWR and block transmission and reception.

2) RG174 coax cannot handle much power. At anything more than a few watts it will melt into a glob.

3) There is probably a better topology like using PC board mounted relays which would present a lower series impedance to the circuit. It may "work" at 10/11M but the VSWR is likely to be high.

4) As mentioned, an interlock is a must. You don't want a transmitter feeding a receiver.

5) You can buy a decent DELTA manual coaxial switch for 50-70 bucks which will have lightning protection.

6) If this is for receivers to share one antenna, a multicoupler is what you seek.
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Old 04-26-2017, 6:56 PM
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Quote:
Originally Posted by mmckenna View Post
I was under the mistaken assumption you wanted to be able to switch all radios in to the antenna.
Not necessarily "mistaken assumption" There will be occasion where
I will have 2 radios on listening to 2 different freq.'s Not very often, but still...

I just have to be very vigilant NOT to transmit while more than 1 radio
is selected as this will certainly release the smoke out of some of the
receiving radio's components.
I will say that I am still "tabling" the idea of some kind of lock out. (maybe)

Quote:
Originally Posted by mmckenna
If there are extra contacts on the relay that won't be used, you could set it up to lock out the other relays.
Not enough. If I had one more contact per relay I could do it.

Quote:
Originally Posted by mmckenna
But, not as much of a challenge, and that would take some of the fun out of it.
I'm in my glory when I'm building something. It's even better when it works!

Quote:
Originally Posted by RFI-EMI-GUY
Why do you have ferrite beads on the center conductors? These will present a high VSWR and block transmission and reception
The ferrite is over the whole RG174 cable, not just the center conductor.

Quote:
Originally Posted by RFI-EMI-GUY
As mentioned, an interlock is a must. You don't want a transmitter feeding a receiver.
As in my answer to mmckenna, I haven't ruled out putting in a lock
to prevent connection to 2 radios. The option that I am toying with now is
to isolate switch 1, 2, and 3. Switch 4 would then have no SO239 and it
would allow 2 and 3 only to connect simultaneously. (Something along
that idea)
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Old 04-27-2017, 3:08 PM
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So, while waiting for the rest of the parts, I got a little
impatient last night. I hooked up some RG58 to either
side of the relay assembly then soldered on some SO239's
and decided to give a shot. Connected my President Lincoln
HR2510, hooked up 12vdc clapped on the relay and I had
receive. No surprise there. Pressed the ptt and looked at the
SWR meter waiting for it to jump to 2 or 3:1. Much to my surprise
the meter barely moved. Checked everything out calibrated the
meter and keyed again. The meter moved less. As I got right up
on the meter I could tell the needle was moving, but barely.
4 watts transmitted just fine, modulation was good, reply
from neighboring station was "sounds great".

I do have an antenna tuner in line so my next test is to
remove the tuner test everything to get a baseline then
hook up the switch box and compare the results.
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Old 04-28-2017, 8:56 PM
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Some changes made. Most notably, single switches
swapped out for a 4 position rotary switch.
Original "Switch 1" may stay in the circuit to turn
on/off 2nd radio connection. (If on, radio 1 and radio
2 will be receiving)
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