WR-703N External Antenna Mod (DIY)

UPDATE: Don’t do this mod, there are better techniques. Example: Solder directly to PCB antenna, or trying the J4 bridge mod as described here: http://blagg.tadkom.net/2012/09/15/better-wr703n-antenna-mod/

Maybe you’ve heard about the TP-Link WR-703N router? It’s a small “pocket”-router with wlan, 1x lan, 1x usb, Atheros chipset, 32 mb ram, 4 mb flash. It can run OpenWRT, and it’s really really cheap ($23 on ebay currently). They require very little power to run (100ma ish) and are powered via a micro-usb connector. And they have great hack potential (come on.. $23 for a linux thingy with lan, wlan, usb, gpio, etc..). You can upgrade the flash memory to at least 8 Mb, and even upgrade RAM to 64 mb if you have access to a hot air rework setup.

I’ve received two, and have just ordered a third, and have quite a few projects I want to use them for. Battery powered mesh network is one, great for getting networking in weird locations, they can run for many hours on a single 18650 battery. Also thinking about using one for a “umbrella warning sign” near the front door (I live in Bergen (Norway), a city with over twice the annual precipitation of the “Rain City” Seattle). A simple laser cut acrylic umbrella logo with the router’s pcb behind – using GPIO to drive a LED if rain is expected the next 8-10 hours.

Ebay link (be sure to buy the blue version!)

The routers have a tiny internal PCB-antenna, so on one of them I’ve added an external SMA connector. There are several ways of doing this, one is to solder the new antenna connector to the top of the existing PCB-antenna. A chinese website (google translate..) suggested desoldering a capacitor “upstream” and connect to the pads it used. I tried this, but will try both to see which works the best later.

Buy a SMA connector on Ebay with pigtail. Pry the blue cover off the router. Extract the PCB. Locate C114 on the backside. Desolder it (easiest is to place the soldering iron parallel to the capacitor so that you’re heating both pads). Solder the antenna core to the pad closest to the flash memory IC. Solder the antenna shield to the other pad (see image above). Cut the existing PCB antenna track.

Use a 6mm drill bit to make a suitable hole for the antenna connector (make sure there is room for the PCB under the connector).

Advertisements

3 thoughts on “WR-703N External Antenna Mod (DIY)

  1. OK guys, sorted this for good, and almost destroying my WR703N in the process:
    Both on-line methods are crap and do not tap the antenna strip the right way.

    1.: The mentioned “easy” method of unsoldering the J4 bridge and putting your antenna cable between the right pad (of the former J4 bridge) and soldering the shielding to the round ground plane pad next to it leaves you with 50-30% quality of signal, the problem is the ground being a digital ground not used by the transmitter (there is some capatitance between it and the analog ground, like the usb shielding metal!!) The antenna signal on J4 is also very weak, so I suspect that J4 is part of a scrubber circuit for antenna matching that don’t carry any real RF power. (I tried the other pad under the former J4, and it was worse!!). So I bridged the J4 pads again with some short wire and tried Method 2:

    2.: “The unsoldering of C14 method”. C14 has some real signal, but it doesn’t look like anything resembling filters in a signal path It’s also next to the 3.3V PSU so it is an antenna scrubbing circuit too and C14 is a low pass filter to filter out some of the lower frequencies as well as being part of the 3.3V PSU. Unsoldering it and connecting my antenna cable across it gave me around 50-60% signal of before. Unscrewing the antenna from my pigtaill actually gave me 70% power, compared to the stock antenna, so that was a clear indicator of this not being the right path.
    So I unsoldered my antenna cable from the C14 pads and picked C14 up from the floor, soldering it back in ! So what to do? I scraped my antenna of the PCB leaving little pieces of copper track on both of it’s connections; (next to the empty left pad for C32 that isn’t soldered in from the factory) and the right side of the antenna, that is grounded to the analog ground. Actually the PCB antenna on the WR703N type routers, resembles the laptop antennas mounted behind any laptop LCD, and laptop antennas are fed through a 50 OHM cable directly from the the wifi card in a laptop, so I had a pretty good feeling that this would be the filtered, impedance matched signal I was looking for. (I was also pretty desperate as my router looked like it had been in a house fire after my failed attempts!): So antenna cable shield soldered to the right side of the 2mm leg left from the PCB antenna nearest the edge of the PCB that is the antenna ground, and the core of my cable soldered to the left pad of he missing C32: Bang!!! 100% signal quality… Yeah!! 34Mbit upstream and 54 down (I have my WR703N configured as a client, so if you has it as an AP speeds are 54 up and 34 down wieved from LUCI, I would imagine).

    The site survey in LUCI give my this:
    Stock antenna; around 7-12 alien networks visible in my wifi neighbourhood.
    J4 mod; 2-3 visible
    C14 mod; 6-8 visible (close to the stock antenna preformance but dropped connections)
    My own mod: 15-21 visible networks!!!! and the highest speed up/dwn I’ve ever seen!

    So this is actually quite an easy mod, as there are no SMD components next to the scraping off, of the antenna, and the hot soldering..

    Please another thing: Just dont stick one of long antennas on your WR703N after this mod (i.e. from an old WRT54G), as they are 5/8 wavelength antennas and don’t match the 50 OHM antenna circuit on your WR703N. The small 1/4 wave 2.5″ kind have better range! unless you get hold of a real high gain 50 OHM stacked whip antenna or something simmilar made for long range wifi outdoor access points.

    I’m not a Openwrt forum member, but this info really belongs there too, to make other HWhackers lives easier! Repost as you like.

    Dr. Luno

    • Hello Luno,
      That is certainly interesting results! I had a go at your mod tonight to compare it to the J4 mod.

      I did 5 runs:
      1,2: J4 mod (with main pcb antenna trace cut)
      3: No antenna (main pcb antenna trace cut)
      4: Soldered to sanded main pcb antenna tracks – rest of pcb antenna cut

      https://docs.google.com/spreadsheet/pub?key=0AqCrA6jj0wgbdEtDdUhWa09TZ3lYWlJUbUtqcTNZSXc&single=true&gid=0&output=html

      1,2: 28 wlans
      3: 4 wlans (as expected with no antenna)
      4,5: 29 wlans

      Signal strength between 1,2 and 4,5 was fairly similar. So I don’t see any big performance changes between J4 mod and soldered to pcb track here :/
      Are you sure the J4 mod in your case had a good connection?

      But, all in all I like your mod, might even be easier to do than J4. And you know for sure that no signal impacting components are left out.

      • Wow quick response, I thought I had posted in a dead thread. about the J4 mod: I have been soldering for 35 years,so yes they were good. I used the same ground as you above the right pad of jumper 4. ( when I solder on ROHS complying solder I first wick up all the non-leaded solder with a wick, which also fluxes the area plenty and then tin the pad with my own old fashion lead solder, not to get a matte and possible cold solder joint by mixing the two kinds). I’m not certified anything RF, but I have built numerous antennas from KHz to 10Ghz including a lot of wifi antennas, my best in 2.4 GHz is a 16 DBi yagi antenna copied from a commercial one with the added refinement of a non counducting boom and a impedance matching baloon at the antenna feed so it actually exceeded the original antennas specs.

        I’m not in fallable, so pehaps I did something else wrong? I tried to trace the circuitry around J4 and I’m not sure that J4 is part of the RF feed. It’s too big a trace and too far away from from the radio output on the chip. The circuitry directly above the antenna looks exactly right as it has coupling and filter capacitors ending in the un populated C32 pad and the antenna feed as mentioned in my post. (a lot of wifi card has an unpopulated pad for a capacitor just before the antenna, Maybe as a requirement in certain countries where filtering has to be better?)

        The fact that I tested my signal in LUCI is actually not the sure way of testing radio link quality, you also have to how good the quality of your signal is at different distances to you to be sure and that takes half a day with a friend on the other end, but my WR703n suddently was the strongest client in my also openwrt router, beating Iphones and MacBooks by 20dB.

        I’m building a robotic rover with the WR703 incased in an alu box and thus need an external antenna and all the signal going to it. The other mods might work as the “coat hanger on the telly”, It increases your reception, but totally destroyes your transmitting power, OK for tv reception, but not for transmitting. So if you stick a piece of wire near the RF circuit, it can pick up more signal and at the same time not disturb the stock antenna too much so you actually get an improvement but I’m sure you get an increased noise level, What we in reallity is interested is the largest difference in dB between signal and noise level, your spreadsheet doesn’t show that. Another trick is to touch the antenna cable. If your signal goes up or down a lot, something is wrong and the cable is radiating i.e. an antenna mismatch to the impedance of the circuit!

        The C14 mod seemed more clever besides the fact that C14 is too big ( I don’t know it’s value, but it looks like a smoothing capacitor for the 3.3v supply right next to it.) So why is it there? I think it acts as a low pass RF filter to filter out lower frequencies. It can also be a part of the antenna or maybe a matching network for tuning the antenna? So yes it carries RF but the stock antenna is still connected. Cutting the stock antenna made the signal worse in my case, indicating that the C14 mod doesn’t do much good. Unscrewing the duck antenna from my short cable, made it better, again an indication that the mod does more harm than good. So I think that C14 is part of a trap for harmonics i.e frequencies close to the transmitting frequency, that have to be filtered not to disturb outside the wifi band. What really should be done if we wan’t to nail it is to have someone with a radio band spectrometer check out how the signal looks at J4 and C14, then we would know their purpose.

        Thank you for taking this seriously, hate when bad advice get’s copied all over the internet, especially regarding electronics!!

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s