Perspective on the Base Station Antenna Board

As usual, the readers of this web site continue to delight me as they manage to find information and pass it along for the benefit of the entire community. Naturally, I'm more than happy publishing it here!

The following perspectives came from several generous readers. If you have a comment of your own, please feel free to contact me and if relevant enough, I will be happy to publish it here. Cheers! - Constantin

One early submission from someone who wants to remain anonymous

The circuit board seems to have a switching circuit that senses the impedance of a external antenna and kills the case mounded antenna amplification but I can't be sure because my eye sight isn't what it use to be and can't determine the part numbers on a lot of the parts and the circuit mask is very dark. Getting old sure isn't fun but it sure beats the alternative.

My ohmmeter doesn't show any reversing of the polarity on the antenna wires in the case. So it should be a safe bet that they didn't try anything sneaky with the Dr. Bott antennas and the certified antenna business unless something is going on in the switching circuit that I am not catching.

James Shields submitted this e-mail from a friend regarding getting a standard antenna to work with Airport Extreme external antenna port

It looks like the external antenna is detected when there is a 50 Ohm DC load on the external connector. To wit: I connect a 51 Ohm terminator to the N connector on the end of my MCX male to N Male pigtail, and cycle the power on the AEBS, and voila, the Airport Admin utility, when it eventually gets connected again (with my iBook right next to the terminator), reports Antenna: External!


So, I remove the terminator and replace it with the antenna, and it seems to work fine, as expected, until the base station is power cycled ot reset for any reason. At that time, the unit reverts to the internal antenna.

So, I guess I need to cobble together a high-pass, N-male to N-female DC terminator thingie. Actually, I think this needs further investigation, either by reverse engineering the antenna board inside the base station or better, a talking to contact inside of Apple or Dr. Bott to tell us what the preferred sensing method is for this external antenna port.

Matthias Pfaff had this to say about his experiences with the Airport Extreme external antenna port

Today I made the MCX to N-plug pigtail - not for the faint of heart, I must say. Sure enough, the base station will not recognize the external antenna just by itself. (Thanks for the tip on your website).

I tried a 62 ohm resistor and a 50 ohm resistor - they both will pull up the external line fine. However, I had to hold the resistor to the little circuit board for the full (!) boot time of the base station (about half a minute), otherwise there is no effect.

I run the base station without the top for the moment as I will have to do the resistor thing everytime power is turned on (otherwise my neighbor will be cut off the internet...)

I ordered a Dr. Bott antenna for my other ABSEx (which is acting as a bridge on the other side of the house). I will then measure the resistor in this antenna and see what solution I can come up with for my main base station.

Editorial Note: A lot of people on the Apple Discussion boards are very unhappy with the actual performance of the OmniAir antennas from Dr. Bott. These issues are apparently exacerbated by the nearly nonexistent literature on the antennas either online or on the boxes they are sold in. Expectations are not properly managed, and the requirement to reboot and set up the AEBS for external antenna operation appears unclear. In other words, using WDS with a Airport Express may be a much better means of extending the reach of your network than adding an external antenna to a AEBS, even if you're adding a Apple-sanctioned upgrade because it's not a plug-and-play upgrade.

Tom Morris on the Antenna Board found in Snow and Exterme Base Stations

I was just reading through your page on adding an external antenna to the Airport Extreme base station...

I noticed the mention of the antenna boards and their possibly containing amplifiers.

Yes, I was indeed bored enough to visually trace the thing out from these images.

Airport Base Station Extreme - External Antenna Board

AEBS with Modem and External Antenna Option

Airport Base Station Extreme - Small Antenna Board
(picture credit: Sascha Großkopf)

Regular AEBS or "Snow" base station

(Click on either image to enlarge it)

The components found in the signal path are either one of two things: one, of course, would be MMIC amplfiers. These would require only a DC bias for power. However, I have yet to see any that are bidirectional, which would be needed for a WLAN application. The second possibility would be that they are simply PIN diodes used to switch that input/output on or off. PIN diodes work by only allowing an RF signal to pass when a certain DC bias is applied. The fact that they are labelled D1 and D2 seems to back this theory up... my guess is, you apply a positive DC bias to set one, and a negative DC bias to set it to the other.

The board from the version with modem looks like it operates the same way... or does it? The same PIN diodes (I'm only 95% sure... honest!) are there, but U1 and U2 are a mystery. U1 seems to handle switching the antenna set in use between external and internal.

I have a couple more theories on that (assuming this board is dual layer, with the back layer being just a ground plane, and all vias tied to ground)...

First, I'm guessing that U1 is a dual PIN diode module.

A DC bias applied to the "Radio" lead would be blocked from entering U1 by C7. However, it can pass through L3 (which in combination with C9 appear to be a lowpass filter, to allow the DC voltage through) and reach pin 5 on U2. The two outer pins on U1 (at the bottom, as seen in the image) are DC only inputs. One takes a voltage straight off the lowermost black wire to the right, and one takes it off U2. U2 also provides the DC bias to the PIN diodes D1 and D2 through L1...

When the base station is using the external antenna, a DC voltage applied to the lowermost black wire applies a bias to the left diode in U1, and U2 provides no bias voltage to the right diode or the internal antenna select diodes. When the base station is using the internal antennas, bias voltage is disabled to the left diode in U1, and the right diode is activated by U2, which can also select the antenna currently in use by applying a positive or negative voltage at pin 1 (leading up to the antenna select diodes).

Exactly what U2 is remains a mystery. My guess is it's some kind of H-bridge line driver, flip flop, or something equally similar - in short, just a switch for logic levels. Pin 7's ground, 14 is VCC... and R3 is a pull-up resistor. My theory of it being a SMD version of the 7400 quad NAND gate wouldn't all work out due to the pinout configuration of the outputs (darn.)

As for the black wires, the top one's ground, next one down's +5 or +3.3 volts, the next two carry logic levels to control the outputs of U2, and the bottom is just a direct input to switch on the external antenna jack.

So... the bottom line... onboard amplification? Unlikely. And, if you're putting some DC grounded antenna (such as a ring Yagi, Yagi with folded dipole element, biquad, etc.) on without going through the existing antenna board (which does have DC blocking already), you'll probably want to put a DC blocking capacitor inline to prevent whatever drivers down on the radio card provide it from getting shorted to ground.

And this is what I happily do when I'm bored.

-- High Prince Crashalot of the Holy Order of Multiball Madness, Suburban Warrior of Disarray, Master of the Lost Circuit Collective, Member of the Federal Narcoleptic Ostrich Relocation Department (FNORD)

Also known to the unenlightened as Tom Morris, KG4CYX