27MHz legacy antenna -- does it work?


My app works using rpsma 2.4GHz rubber ducky antennas on a 900HP s3b module indoors up to (and perhaps more than) 100 feet. I need the modules to talk to each other about half mile apart with LOS.

There are two 27MHz whip antennas installed at the site already (they look about quarter wave length – approximately 2meters long) where the app will be deployed. I tried to use them with an rpsma adapter and some bnc cable. It didn’t work.

Although the antennas are not designed for the s3b 900MHz frequency, I’m not sure why they wouldn’t radiate, given enough power. After all, the modules seem to radiate fine with a 2.4Ghz rubber duck, but this is much shorter, just a 26mm quarter wave.

It seems like this is a power issue and not a frequency issue. The radiation resistance of the dipole being proportional to the square of the characteristic antena length would drive up the power required considerably. Maybe it’s both a power and a frequency problem.

Here is my question. Do I need to buy a 900MHz antenna or do I need to buy a booster or both?
I want to buy this one:


Is that going to work? How much bnc cable can I add without fear of signal attenuation? It would be nice to have about ten feet between the end of the antenna and the modem, but I could make do with less if necessary.

If I do need to boost the modem’s output, can someone direct me to a product I can solder on to a protoboard?

I’m pretty new at this, so if these questions are dumb don’t hesitate to tell me and explain. Thank you.


You need top purchase the 900 MHz antenna as that is what the FCC and other regulatory agencies approved that radio with. Anything else and you are out of compliance and can be forced to shut down and or pay fines.

Thank you for your response. I’m still broadcasting (or trying to) at 900MHz, so you’re saying the FCC would fine me for broadcasting in an accepted frequency band with the wrong antenna? Sounds a bit like the mattress police.
Anyway, I’m happy to shell out the extra 120$+ for the patch antennas referred to in the link, but only if they work. If this is a power issue and not a frequency issue, as I alluded to in my question, then the large patch antenna (ideally with 10 feet of bnc cable) won’t radiate either. It will need a booster, yes? Do you, or does anyone know the answer to this? If yes, is there a 900MHz booster that can be soldered onto a proto board or something?

No you can’t use that antenna. You need to use one of the ones listed in the approved antenna list or similar as described at http://www.digi.com/resources/documentation/digidocs/90002173/Default.htm#reference/r_certs_xbee_900hp_antennas.htm%3FTocPath%3DS3B%2520hardware%2520certifications|Agency%2520certifications%2520-%2520FCC%2520(United%2520States)|_____1

IF you use the right antenna and you have RF LOS conditions, you should not have an issue.

James, do not use these antennas with a 900MHz device. Antennas are tuned to the frequency they can efficiently radiate and have the gain calculated appropriately.

What happens when you send a signal that cannot be radiated properly from an antenna is that it is reflected back to the transmitter. This, apart from obviously not sending an optimal signal, can destroy the transmitter. https://en.wikipedia.org/wiki/Standing_wave_ratio

A 27MHz quarter wave is 2700 cm. Even a full 900MHz wave is 33.3 cm. If you really want to see how out of tune it is put a SWR meter on it.

Change your antennas and preferably change the coax.

As mvut said too, if you start broadcasting poor or overpowered signals that affect other devices you will feel the wrath of the FCC.

Hi Nicholas, and thanks for your response. Yes, somehow I managed to avoid destroying the modem with my little 27MHz experiment. Luck! What are your thoughts on the patch antenna link I included? Any reason why that 900Mhz patch won’t work? It would save me a few bucks.

Link again for convenience:


Again, thank you.