Sharing an antenna between RF modules

[iter]

I am building a PPM encoder. I plan to reuse an RF section from a throwaway 72 MHz TX. I also want to use the same encoder on 27 MHz.

I know Futaba, Hitec, etc. radios that use RF modules use the same antenna for all frequencies. I imagine that it's suboptimal for 27 MHz, but it seems to work.

Now I want to go one step further. I want to permanently mount both modules in my TX and have the software select one of them. Each model memory knows which frequency to use, and enables the right module.

I can have the uC turn Vdd on or off to each RF module. I wonder if I should somehow control their connection to the antenna, too? What happens if I have two modules connected to the same antenna, one powered on, one powered off? Will RF from the "on" module do anything bad to the "off" module? Is there a kind of transistor I can use between the "RF out" pin on module and the antenna?

Ari.

[jeffs555]

Having two RF modules connected directly to the same antenna at the same time would be a very bad idea. Even though the power is off to one module, the passive components(capacitors and inductors) will still be connected and will detune the antenna. At best, it just wouldn't transmit very well. At worst, it could destroy both modules. While you can use transistors to switch the RF, the design of a good RF switch is not trivial. The easiest solution would be to use a small relay to switch the antenna.

PS. I have to add the standard disclaimer that what you are trying to do is illegal as far as the FCC is concerned.

[iter]

Having two RF modules connected directly to the same antenna at the same time would be a very bad idea. Even though the power is off to one module, the passive components(capacitors and inductors) will still be connected and will detune the antenna. At best, it just wouldn't transmit very well. At worst, it could destroy both modules. While you can use transistors to switch the RF, the design of a good RF switch is not trivial. The easiest solution would be to use a small relay to switch the antenna.Thank you for your explanation and the relay suggestion. Can you recommend a relay type? Would solid-state work? Also, can I connect power and antenna with the same DPDT relay, or should I have them on separate connections so that the antenna connects first and then power comes up after a small delay (after contacts stop bouncing)?

PS. I have to add the standard disclaimer that what you are trying to do is illegal as far as the FCC is concerned.I understand that, but thank you for mentioning it anyway. I add my own disclaimer--this question is for my education only, "want" != "will" etc.

Ari.

[Tomapowa]

...

[Tomapowa]

Definitely possible (and legal!) ... that's what "diplexers" are for...

http://www.radio-electronics.com/info/antennas/diplexer/diplexer_concept.gif

http://www.radio-electronics.com/info/antennas/diplexer/antenna-rf-diplexer.php

http://books.google.com/books?id=14_yXDVs2_gC&pg=PA41&lpg=PA41&dq=diplexer+diy+formula&source=web&ots=5GzGKaCBC-&sig=er7pfgXqQEWW2kyaICIZcWndMP0&hl=en&ei=mqqQSZ2bOpjeM_L_lbAL&sa=X&oi=book_result&resnum=4&ct=result#PPA40,M1

Here's a cool looking DIY Diplexer app... have not downloaded but sounds cool:
http://www.softpedia.com/get/Scheduling/DiplexerDesign.shtml


BTW, don't know if any one else posted this previously but this is a great source/guide on Antennas and RF propagation...
http://www.scribd.com/doc/299766/USMC-Field-Antenna-Handbook

[iter]

Thank you Tom.

This looks very interesting, but beyond my current level of understanding. My interest is primarily in programming my encoder. My knowledge of RF and even EE is limited.

Ari.

[mjsas]

You could try a solid state relay but who knows what's inside of one of those things. A small mechanical relay should work best.
Since the output power of a transmitter is limited by the FCC any crappy antenna can be driven hard enough to reach the FCC power limit. I suppose they could use a coat hanger for an antenna and drive it hard enough to reach the FCC power limit. On the receiver end the antenna gain needs to be optimum.
Transmitter modules can be pretty fussy about antenna loads.

[Tomapowa]

Transmitter modules can be pretty fussy about antenna loads.
Yes they can, in fact, most RF output sections (high power apps mainly) can get damaged if not properly terminated with an impedance-matched load, whether an antenna or dummy load. So, to use a relay (mech or elec), use a double pole/throw version such to switch each output section to either the antenna or dummy load.

[iter]

What makes a good dummy load? A resistor to the ground?

Ari.

[Tomapowa]

What makes a good dummy load? A resistor to the ground?

Ari.
Correct... as long as the resistor's wattage matches that of the output stage.
http://www.mycal.net/old/projects/mpr/powerm.htm

[jeffs555]

As long as only one module was powered at a time, you would not need a dummy load. A simple SPDT relay would work. Shouldn't have to worry about sequencing the power to the relay and the modules. If you get a relay the same voltage as the voltage to the module, you could just connect the coil in parallel with the power to one module. When that module is on, the antenna would be connected to it and when off it would be connected to the other module.

[iter]

Tom, thank you for the link. I'm not sure I follow the schematic in it. I am not clear where the author connects his RF output and where would he connect his antenna if he weren't using a dummy load.

Jeff, thank you for your idea of parallel coil and power to RF module. What I was trying to prevent with a timing delay was a situation where a module is already powered up but is not yet connected to the antenna. If this is a problem, your parallel connection idea is likely to make that problem worse, not better, because a FET (that works the coil and the RF power) opens faster than the relay closes and there is a guaranteed time delay between RF module power-up and antenna connection.

Ari.

[tune by tito]

you will need a RF general purpose single-pole, double-throw switch device designed for mobile communications applications such as cellular and PCS mobile phones, ISM bands, GPS receivers, L-band satellite terminals, WLAN and pagers.
Digi-Key Part Number 779-1009-1-ND price $1.00 qty=1
• SPDT Switch for mobile communications:
o Insertion Loss: 0.4 dB @ 2.0 GHz
o Isolation: 20 dB @ 2.0 GHz
o VSWR: 1.3:1
o P0.1dB: +26 dBm
• RF-frequency range DC – 2.5 GHz
• No supply voltage
• Positive control voltage
• Small SC-70 plastic package

when you switch a normal relay on and off the relay will induce a noice + the 27-72Mhz modulation in the antena creating a long amplitude modulation as long as the relay is bouncing(spring back) creating deformation of the original signal.
best regards tito

[jeffs555]

I doubt you will have to worry about the few milliseconds of time between when the transmit module is powered on and when the relay switches or about the bouncing of the relay. People range check every day with the antenna collapsed and while that is not exactly no antenna it is severly detuned and lasts for a lot more than milliseconds.

[ZAGNUT]

a lot of the ACE radios had/have more than one RF module installed, sometimes a mix of AM/FM and sometimes on different bands. a toggle switch is used to switch between the decks. i'm pretty sure this only switches power, ground and PPM which would mean that both antenna leads are connected all the time.


dave

[Tomapowa]

....which would mean that both antenna leads are connected all the time.


daveNot directly I bet... most likely both RF output sections are connected to the same antenna through a diplexer/RLC network of some sort. Pushing RF into another RF transmit section is not a good thing to do... or attempt.

[iter]

Tito, thank you for posting your datasheet. Looks like exactly the kind of switch I need.

I wonder about blocking capacitors. The DS says, "Optimized performance can be achieved <0.5GHz by increasing blocking capacitor to >100pF. Optimized performance can be achieved >2.0GHz by decreasing blocking capacitor to <100pF." This device is primarily designed for frequency ranges that are one or two orders of magnitude higher than our TXs'. What is a good capacitor for 72/27MHz?

Incidentally, I've never seen a coax cable antenna connection in a hobby TX, always plain multistrand wire. Is that because at lower frequencies cable is not as important or have I simply been looking at cheap equipment?

Ari.

[barc]

Use something like 4700pF cap as the blocking cap.
At 72/27MHz range, you can just use a small SPDT relay.

Chris

[tune by tito]

iter
the blocking caps are for stoping the switching DC voltage to enter the AC modulation path and AC to DC, the cap are not use to filter the 27-72Mhz noise or else, they are use only to block DC or AC voltages you can use the recomended cap or try other (lower)values for you application!!!!!
also you not need a coax cable connection, you can connect the antena output very close to the switch in you pcb. also you need to take in consideration the amount of milli watts you TX output, for 27Mhz max is 4 watts, for 72Mhz is less than 850 milliwatts, most Tx output less than what the law permit!!!!! check FCC documents report to know how much you unit output "if a comercial TX", is more than 1 watt you will need a more powerfull RF switch,
search Digikey or Mouser for RF switch, thay have lots of them which can work with you application.
best regards tito

[mjsas]

Incidentally, I've never seen a coax cable antenna connection in a hobby TX, always plain multistrand wire. Is that because at lower frequencies cable is not as important or have I simply been looking at cheap equipment?Ari.
At 72 MHz the wavelength is long compared to the bit of wire so it does not matter. In any case only the radiated power is the only thing that is of concern, not how it is done.
The textbook design is a 1/4 wavelength monopole antenna over a ground plane. At 433 MHz the monopole would be about 6" and a cookie sheet would make a good ground plane. At 72 MHz it is not practical to use a ground plane so they just jack up the drive voltage. In any case the antenna impedance is not 50 ohm so it does no good to use coax cable.

[iter]

At 72 MHz the wavelength is long compared to the bit of wire so it does not matter.Ah! I see now. Thank you for clarifying that.

I learn so much from you guys. I feel gratitude to you and this community.

Ari.