AVM6842 with DC shorted external antennas - modification

In a thread on the german lte-anbieter.info forum it was noted that the 6842 does not work with DC-shorted antennas, and that according to AVM, it is not supposed to.

I took WLAN modification cables that I had

http://www.wlan-shop24.de/FritzBox-Spee … aeuse-Clip

The IPX connectors appear to fit

The antenna selection on  the Zugangsdaten page should be set to internal antenna. Otherwise it won't work.

Now a DC shorted panel antenna 1800 MHz

http://www.wlan-shop24.de/LTE-1800-MHz- … door-17dBi

gives the same result with the 6842

as with the 6840 yesterday

The same antenna would still give some signal to the unmodified 6842 albeit much weaker (ca 15 dB) with the setting external antenna . AVM specifies that DC shorted antennas should not be used with the 6842. Well, most high gain antennas are DC shorted for various reason, statics and lightning protection among them.

The alternative is using 30 € each adapters with built in capacitors. However, these adaptors don't fit well on the router.

You will obviously need RP-SMA to SMA adapters with the modification set for WLAN. I had some at home that I used.

It would be preferable to use shorter IPX/UFL to SMA pigtails and to mount the SMA connectors on the top of the router so that paddle antennas could be used in good positions. I tried 1800 MHz rubber duck antennas that did work on the 6840, although the gain is much smaller than with the panel antenna.

The gain of the rubber ducks, as shown below, is the same on 1800 as the original 6840 paddle antennas. Note the polarization effect between Rx0 and Rx1, each receiving one of the Tx antennas with better SINR than the other. The mast is straight ahead at 5,6 km as shown in the router. There are no neighbor cells that interfere.

Here are SMA pigtails that might be useful: 15 cm long.

https://www.adafruit.com/products/851