1
\$\begingroup\$

I moved to an offgrid house with a 3G/4G repeater, with a short, enclosed directional antenna. It was wobbling and poorly mounted on an unstable wood plank at the base of a window, so I decided to fine-tune its placement.

I checked maps and azimuth for nearby public GSM antennas. The only realistic, nearby one (at about 2km) is indeed right in front of the window, however there is a small hill in between. Others are much farther and way under the horizon.

To my surprise the best result I got is when my antenna is the closest to the window glass, and more surprisingly, my SNR even improved when I placed it parallel to it, pointed towards the concrete side wall of the window, not towards the GSM pole which should be pretty much at right angle.

The concrete probably has rebar so it might create a loop, but I imagine it would certainly be grounded.

This really makes no sense to me and I am looking for an explanation (beyond black magic.)

My only guess is that the triple-glazing wooden framed window might be metal-coated, and it would "amplify" the signal, that the directional antenna catches laterally? Does this even make sense?

I know just enough about radio engineering so as not to trust myself too much and keep a low profile, so I also cowardly checked every angle with the antenna mounted on a tripod on my terrace (same height as the window,) with 5° increments.

No way, the best placement is the one shown on the picture.

  • AFAIK it is an LDPA antenna since it catches may wavelengths, not a Yagi.
  • The coax cable between the repeater and the antenna is too long. It was stacked vertically along the frame. Worse, it was originally coiled. I use folds instead to avoid loops, but I am not sure it has any impact on the direction (only noise/loss, which I admit was barely noticeable.)

directional antenna weird pointing direction

\$\endgroup\$
8
  • 2
    \$\begingroup\$ Triple glazed windows typically also have a metallized coating to reflect heat. It will work quite well as a reflector for radio signals as well. \$\endgroup\$
    – JRE
    Apr 8, 2022 at 8:52
  • \$\begingroup\$ Antennas do generally not fare well when placed in parallel with something containing metal. That includes armed concrete and in some cases possibly the windows too. Ideally it should be pointed 90° away from all such things that could act as ground planes. So your current antenna placement is indeed far from ideal, but perhaps this position puts it more out of the way from the concrete than the previous location. \$\endgroup\$
    – Lundin
    Apr 8, 2022 at 9:54
  • \$\begingroup\$ Unrelated to your question, at a glance your connectors don't look IP proof(?) and if so shouldn't be used outdoors. \$\endgroup\$
    – Lundin
    Apr 8, 2022 at 9:54
  • 1
    \$\begingroup\$ If I recall correctly, the sweet spot for a flat reflector in Yagi and similar designs is about lambda/6th (placed rear, spacing from the active dipole). Which gets more difficult to grasp if you consider multiple frequency bands... And, adding a "braindead sideways" reflector may indeed completely re-shape the radiation diagram of your antenna. Loosely in that vein, bare LDP antennas have no impressive front-to-back ratio... As for "but this is the best result I get in practice" - I am not surprised, and I agree to believe what you've verified by experiment and what works for you. \$\endgroup\$
    – frr
    Apr 8, 2022 at 10:17
  • 1
    \$\begingroup\$ Coiling the coax vs laying it flat should have absolutely effect on your signal, so long as when it's coiled the loops are not so tight that you risk kinking the cable. (Radius of the loop) > (the diam of the cable) is good layout practice. \$\endgroup\$
    – SteveSh
    Apr 8, 2022 at 11:37

3 Answers 3

2
\$\begingroup\$

My guess it that your thick concrete wall opening backed by window glass, is acting as a cavity-backed aperture antenna. This type of antenna is more conventionally designed to be sharply resonant, with highly conductive metal sides and back. However, in your case, the reinforced concrete and metallized glass are lossy materials, and the dimensions are several wavelengths long, so it's not so narrowband, but the large aperture helps to make up for the inefficiencies.

As others have pointed out, your antenna is a short log-periodic and has very modest directivity. It's really just functioning as a broadband dipole, so pointing it sideways rather than towards the GSM tower doesn't cause much loss. I think you stumbled on (or, by subconscious genius, selected) a good location for this to serve as an exciting element for the large cavity antenna.

If you're up for more experimentation, you could try moving the antenna up and down along the side of the window aperture.

\$\endgroup\$
2
  • \$\begingroup\$ Nice, it starts to make sense to me ! But for the non-collinear placement of the antenna, it would act a bit like a super wide and short cantenna ?! Actually I did try to slide it sideways towards the outside but it was degrading the signal. Idem when I moved it closer to the glass. Now I am thinking about: 1) trying to hang the antenna so it is on the axis of the window 2) lay aluminum foil down on the concrete to make it more resonant (not sure about grounding) 3) may be swap my pseudo-LDPA for a bare dipole - or just remove some guides within, so it might catch more lateral signal \$\endgroup\$
    – MoonCactus
    Apr 8, 2022 at 17:15
  • \$\begingroup\$ @MoonCactus Right, good thinking, you can imagine it as a short, fat, square cantenna. If you want to pick up a few more dB, move your repeater closer so you can use a shorter, high-quality low-loss coax cable. \$\endgroup\$ Apr 8, 2022 at 17:56
3
\$\begingroup\$

Given the complexity of your real life problem, I don't think a simple answer will be possible.

But indeed close to metal object, reflections due to the metallic environment will modify the far field pattern of your antenna. In fact, now your antenna isn't just the antenna itself but all the metallic object around it.

If the subject is really ticking your curiosity, you could replace the actual antenna by a dipole and try checking multiple positions to see if one seems better if any.

I'have already seems antenna coupling to rebar, or metallic frames, which seems to act as director, and reflector of a yagi antenna.

\$\endgroup\$
3
  • \$\begingroup\$ Yes, real life is the problem I stumble upon each time I get close to radio-frequency, lol. I am unsure I would buy various antennas just to check in hit and miss mode. Actually I could try a cheap pizza pan antenna at least... \$\endgroup\$
    – MoonCactus
    Apr 8, 2022 at 8:31
  • \$\begingroup\$ @MoonCactus or you could look for a scrap satellite dish :-) but that won't help you if you're actually facing a problem with an oscillating repeater (feedback = not enough separation between inside/outside antennas). \$\endgroup\$
    – frr
    Apr 8, 2022 at 8:35
  • \$\begingroup\$ Indeed a satellite dish could enable to counteract the effect of the coupling with rebar. To be tested ! \$\endgroup\$ Apr 8, 2022 at 12:04
2
\$\begingroup\$

If you're using the antenna with a repeater, consider a different problem: what if the repeater is picking up feedback? Perhaps it's clever enough to detect this condition and decrease its gain... The more separation you provide between the inside and outside antennas, the better "performance" you percieve, because the more gain the repeater can reach without oscillation.

Repeaters are double edged :-) Some may cause periodic "outages" in your mobile phone signal, depending on internal construction and on the level of separation between inside and outside antennas. The repeater keeps slowly increasing its gain, and when it detects feedback (instant overdrive) it drops the gain stepwise significantly - starting a new cycle of gradual adjustment until the echo kicks in again, ad infinitum.

Any chance for you to try e.g. RTL-SDR on the bare antenna, without the repeater active? It is still possible that your antenna placement in front of a reflective window is indeed optimal in terms of signal gain towards the tower - but until you try to actually measure something, you will be no wiser. BTW, RTL-SDR is only any good up to maybe 1800 MHz, but the 2100 MHz and higher bands are definitely out of reach. Are you perhaps able to find out, what particular frequency band your phone is picking up? Do you have access to a "stand-alone" GSM/LTE modem by any chance?

GPS or GSM/LTE repeaters are a plausible choice on buildings that have a sheet metal roof (or are coated all around by sheet metal). Like a mountain house with a rugged metal roof or a warehouse/hangar. Even then, any analog repeater principally adds some noise of its own (which can be alleviated by optimal antenna placement) and making a repeater work for multiple bands adds complexity and cost (starting from the design stage). You may find that your off-the-shelf repeater does not support an interesting frequency band that your devices would otherwise choose etc.

In a particular situation = a mountain top with a number of towers visible in a distance, the phones would pick up a great number of "neighbour cells" at extremely weak levels (-120 dBmW) and relatively plausible SNR. And the network would hint the phones to use the higher, urban bands (2100+ MHz) from the towns in the valley, as these have more data capacity and are less occupied by devices. One of the people in the mountain village got a repeater, which reportedly helped him - the phones stopped seeing the weak remote signals (obscured by the repeater's own noise?) and started clinging to the stronger signals in the lower bands (900 and 800 MHz) which was desired from "call stability" perspective. The undesired effect was "pulses of feedbacked interference" due to repeater AGC, as debated in the comments. The wooden house just did not provide enough separation between antennas, in spite of having a metal roof (and yes the outside antenna was placed inappropriately, on a wall).

\$\endgroup\$
7
  • \$\begingroup\$ Interesting hints ! My repeater shows steady "full signal" bars on GSM/3G/4G but I admit I saw no significant, actual improvement to bandwidth on the mobile phones. There are a few outages per hour at worst so I doubt AGC is the culprit. But still, I should try and move the repeating antenna further apart from the device and from the outside antenna (~5 meters now). As of the tools, I fear my USB SDR is too cheap indeed for these high frequencies. My archer MR600 4G modem might help, but I guess it relies on its two rabbit antennas instead, since I improved bandwidth by moving it outside :o \$\endgroup\$
    – MoonCactus
    Apr 8, 2022 at 9:35
  • \$\begingroup\$ @MarcusMüller of course. Without filters, it would be a complete no go, because you'd get the feedback coupled at the ports of the duplex repeater (no need even for antennas ;-) The way that the up/down signal paths are split by some sharp filters (hard to grasp for me how steep they must be) does not in any way preclude the repeater from picking up feedback between its two ports via antennas, as each half of the duplex path is just directly forward-amplified = no mixing / freq.conversion. Not with the cheapest repeater variety. Now imagine some are even multi-band... \$\endgroup\$
    – frr
    Apr 8, 2022 at 9:38
  • \$\begingroup\$ The Archer is actually a router. It does contain a 4G modem of some sort - without further research I cannot tell the brand and model of the 4G module. As this is a router, you cannot get your hands on the modem's AT command interface = your detective options are limited by your router's HTTP GUI (firmware). Perhaps if you'd install OpenWRT, you might get access. The bare LTE modems do tend to have some detailed information at their Hayes-style AT commands interface, can be configured to avoid some bands etc. Useful when your tower makes your modem prefer a band with poor signal. \$\endgroup\$
    – frr
    Apr 8, 2022 at 9:46
  • 1
    \$\begingroup\$ And yes my preferred way of tackling GSM/LTE repeaters is by managing to get by without a repeater :-) Giving a better signal to your LTE radio, by way of placement or a better antenna, is certainly the right way to go. You can try to elevate your antenna as high as possible, but consider adding a coax surge arrestor. \$\endgroup\$
    – frr
    Apr 8, 2022 at 9:49
  • \$\begingroup\$ Repeater: yes this is a 3-band amplifier I saw on aliexpress for about 60€ IIRC (looks like this fr.aliexpress.com/item/1005002345654888.html), it is somehow hard to tell how good a job it does tbh. About the 4G router, its web-based IHM does show the RSRQ, RSRP and SNR and it lets me lock it on 3G or 4G. I am not sure what more info I would get with AT commands. \$\endgroup\$
    – MoonCactus
    Apr 8, 2022 at 9:59

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.