Need help tuning setup - block back side of antenna

[goyop]

I have a cell signal amplifier with a dual band yagi on the front end and a panel antenna inside the building. The problem is that I cannot get enough boost from the amp for the inside antenna due to oscillation or feedback back out to the yagi. As it is I had to detune the yagi to get any gain at all inside.

To clarify, following is what the system does:
1. Yagi is pointed to cell tower to send and receive
2. Signal from yagi runs to a Wilson dbPro dual band amp
3. The second internal antenna is connected to the other side of the Wilson amp and this side has a gain adjustment. If the gain is too high and causes feedback, the amp shuts down. The inside antenna is about 50 feet from the yagi and is aimed at the backside of the yagi.
4. When I try to get a stronger signal from the internal antenna the amp shuts down. So I cannot get enough signal.

The antenna placement and the direction they are pointing are optimized as much as the facility allows. My idea was to completely block the backside of the yagi to prevent the inside antenna from hitting it when I turn up the gain. So I was thinking to do this I would create a simple shield like a faraday device with metal mesh that is then taken to ground.

Attached is a JPG to clarify the setup.

Does anyone see any problems with doing this? Likelyhood of working? For the size I was thinking four feet by three feet with the yagi more or less in the middle. Is that large enough? Other suggestions?

Thanks all.

 

[jiripolivka]

In similar systems the main problem is how to achieve a good isolation between the two antennas so you can use some gain.

A Yagi antenna has usually the side lobes that degrade such isolation. Inside of the building you must try to prevent the direct signal coverage to exist, otherwise the system will oscillate as you experienced. Using a Faraday cage may help to screen the indoor room. Again, as the cell phones may move in the room, the conditions may and will get worse.

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My question is how to make sure one can transmit and receive through unidirectional amplifier between your antennas?

Instead of trying your solution, several years ago a "microcell" solution was introduced, with a secondary base station located in a service room, and external antenna communicating with the closest "tower" base station. This does work and no feedback is observed.

 

[goyop]

Thanks for the assistance. I will go ahead with isolating the back of the yagi from the signals that may hit it from the inside directional antenna.

The amplifier I have is bi-directional. But we are still having the feedback due to not having the yagi isolated enough. So I will report once I have a fix installed.

Question - I am planning on placing some galvanized wire mesh directly behind the yagi and grounding it. I was planning on something 4 feet wide by 3 feet tall with the yadi in the center. Any suggestions or does this seem sufficient?

Thanks

 

[jiripolivka]

I do not think your solution can work. The problem is that the room is not well screened from your external Yagi antenna. This causes the feedback.
How can you use a "bi-directional amplifier" with one antenna at each end? Such system MUST oscillate in any situation. The only way to stabilize your system is to use one amplifier one-way with one receiving antenna and another to transmit.

You may not know but any cell phone does not receive while transmitting but antenna is switched to receiver or to transmitter, othewise the phone would oscillate.

This problem has been solved by the "microcell" base stations as I mentioned. Your system cannot but oscillate.

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Another possible solution to your problem is to use a passive reflector (like a billboard) focusing signals from/to a base station to/from the cell terminal in the room having no line of sight. No amplifiers.

 

[E Kafeman]

For the external antenna, use something with less back lobs such as a parabolic dish. As also antenna gain can be improved with a better antenna, do it require less amplification for same result. Do not use antennas for both 850 and 1900 MHz. If the system oscillates at 850 MHz will it kill 1900 MHz, and if 850 MHz never is needed, is it better to remove one half of the problem.
For indoor antenna is it better to split the antenna in several antennas, where each antenna only have a local coverage, or even better, use same solution that is common in road tunnels and subways, lossy cable as antenna. That way can a big area be covered while it still maintain low local radiation levels, hopefully to low levels to cause feedback.
Increase distance between indoor and outdoor antenna, place outdoor antenna above roof-height as roof adds losses. Several ferrite tubes around coaxial cable at both sides of the amplifier can prevent local feedback caused by reflection losses. Ferrite tubes near antennas does at least not hurt.
Covering outside wall/roof with absorbing material is possible but expensive. Metal mesh as improved reflector behind outdoor antenna can change something to the better or worse. Feed that mesh with a long ground wire can be a low frequency thunder protection, but at 850 MHz will result be very minimal.

 

[goyop]

To clarify a few points, the sytem I am using is a "micro cell" as mentioned.

Instead of trying your solution, several years ago a "microcell" solution was introduced, with a secondary base station located in a service room, and external antenna communicating with the closest "tower" base station. This does work and no feedback is observed.

There is still feedback when I try to get sufficient gain from the internal part of the system. That is why I am thinking to block the rf from the back of the yagi. The amplifier has an automatic feedback suppressor so if I try to get the gain I need the amp shuts down.

These systems are commonly used to bump up the signal in a building that is not receiving a sufficiently strong signal. They are also bi directional. My understanding is that (I may be mistaken) cell radios transmit on one channel and receive on another so that is not the cause of feedback or oscillation. Cell phones are truly bidirectional. And the amp I am using is designed specifically for bi directional cell communications at multi frequencies to cover both cell bands.

So I have built an isolation screen of 4 feet wide x 3 feet tall. I will mount it directly behind the yagi and connect it to rod that is driven into the earth by way of #6 copper wire. This should take care of the need for the screen to be grounded to block rf from the back side of the yagi and hopefully help with any lightning. (We never had lightning in this area but it still needs covered.)

I feel confident that this will improve the situation but will post results.

Questions - On a parabolic antenna like mentioned, is the parabolic dish a ground plane? If so I have an unused parabolic reflector that could be used as a simple rf blocking device behind the yagi. Although I will probably try the grounded mesh first since it is already constructed.

Question 2 - You mention

Feed that mesh with a long ground wire can be a low frequency thunder protection, but at 850 MHz will result be very minimal.

What result are you referring to? The blocking of RF in normal operation or somthing related to storm conditions?

Thanks all

 

[E Kafeman]

What result are you referring to? The blocking of RF in normal operation or somthing related to storm conditions?

Lightning due to thunder have its main discharge energy in low frequencies ranges (<30 MHz). At low frequencies can a metallic plate be assumed to be grounded reasonable well, if the wire not is too long.
Assumed maximal length to make any real difference, is about one tenth of the wavelength.
At 850 MHz, is that 20 mm.
At DC can you use 20 meter long grounding cable, and it will work perfectly to reduce electric fields close to the wall. However, a few meters away will it not make any measurable difference in any direction, electric fields will just as strong. It is just not possible to build a Farradays cage with just one wall and from RF view is it still very ineffective if only 5 of 6 sides in a metallic cube is tight connected. The sixth wall makes whole difference. Inside the cage will electric fields be reduced. It does however not matter if the cage is grounded or not.
Compare with a car-body that protects against electric fields due to lightning, even if it have rubber wheels.

Back to your wall:
Assume that you manage to connect that wall to a stable RF ground with a real short cable.
Next problem is that your 850 MHz signal will not care much about that grounding.
The plate will still be just as good reflector or re-radiator, grounded or not. Connecting a metallic part to ground is not automatically a RF-killer, compare that many types of antennas are by design directly connected to ground by a part of its structure.

The parabolic dish works much as the reflector in car headlight. The radiator (dish head) corresponds to the bulb. Big question is, will a headlight reflector be improved if you connect it to ground? Same for the parabolic dish.

I am using is a "micro cell" as mentioned.

Can't see that and a Wilson dbPro dual band amp is not a a micro cell?

 

[goyop]

OK I have installed a 4 foot x 3 foot grounded mesh behind the yagi antenna. It works perfectly on the 1900 MHZ band as now I can push the gain on the amplifier to almost the maximum. However, as you predicted, it has less improvement in the 800 MHZ band. Since I don't care about the 1900 MHZ I reduced the gain to the lowest setting and this allowed me more gain on the 800 MHZ with a noticeable improvement in "bars" on the phone from the normal 1-2 up to 2-3. It went from -75/-80 to -65/-70 We still need a bit more though.

Can you explain to me why the barrier blocks the 1900 MHZ so much better than the 800 MHZ? I know it is twice the wavelength but I am uncertain why the grounded mesh works less effectively in that range.

Regarding the micro cell issue, I only mentioned it because one person somehow thought I was trying to use a unidirectional amplifier and suggested a micro cell. My comment was to clarify that the amplifier is bidirectional and acts like a micro cell within the reach of the indoor antenna.

FIXES?

The internal antenna is as far away from the external antenna as is possible in this setting.

The external antenna is only a few feet higher than the internal antenna. How much higher do you think I would need to go to achieve more isolation from the indoor antenna. The indoor antenna is oriented to the horizontal. I requested a coverage map but what they sent me is impossible to read. The horizontal spread is about 160 degrees and the vertical looks to be fairly narrow. What is your guess for height if I want a 10 db improvement on isolation?

I ordered some ferrite cores and will see if that helps.

Looking at some parabolic grid antennas for 900 MHZ and the back lobes do not look any better than the yagi. I don't really need a hotter signal from the tower, I need more isolation from the indoor antenna to the outdoor antenna.

Any suggestions are appreciated. Thank you for your patience and advice.

Following are some specs on the amp. They do not have a data sheet available for the amp only general specs.

Wilson DB Pro cell phone signal booster with +65dB gain, is designed for homes, offices, fire stations, command posts, and buildings with multi-room coverage requirement. Paired with Wilson's external and internal antennas, the DB Pro amplifier can cover homes and buildings with a 5,000 square feet area, depending on outside signal and other factors.
• Supports multiple devices simultaneously
• Oscillation (interference) detection
• Supports all carriers that transmit at 824-894 and 1850-1990 MHz frequency bands and works on all generations of CDMA, TDMA, GSM, and 3G
• Amplifies signal both to and from cell towers


Frequency 824-894 MHz / 1850-1990 MHz
Gain 62 dB / 65 dB
Max Uplink Power 1200 mw / 1114 mw
Max Downlink Power 26 dBm / 25 dBm

 

[goyop]

Question - do you see any benefit to mounting the two antennas "back to back"? The maximum horizontal distance I could manage is only about 30 feet with maybe 6 feet of vertical separation. Thoughts?

 

[FvM]

The technical specifications of the Wilson DB Pro are a bit vague regarding uplink/downlink sepearation. The "large areas" Wilson amplifiers have configurable channel filter options with distinct separation, but it's not clear what DB Pro does in this regard. Under certain conditions, the cable bound round trip isolation may be a problem, and if so, it will also depend on antenna VSWR. So the question arises if the dual-band antenna is specified for operation with DB Pro?

But more likely, it's a problem of over-the-air feedback. I think, the DB Pro installation manual gives reasonable suggestions for usual sites. It seems to utilize primarly the minima in radiation characteristics of both antennas. It does not promise to operate the amplifier with maximum gain at any site.

I can imagine that cable sheat waves might be a problem, so ferrite tubes at both cable ends won't be a bad idea.

do you see any benefit to mounting the two antennas "back to back"?

I think better not.

 

[goyop]

Thanks for thinking it through. The DB Pro is designed specifically to work with a dual band antenna FYI. However I have the 1900 MHZ gained turned down since this seems to allow more gain before feedback on the 800MHZ.

I did see an improvement with my shielding but not enough. So next will be the ferrite tubes.

I do NOT have the indoor antenna pointed downward from the ceiling. The amp I purchased is for large buildings and they allow for the indoor antenna to be wall mounted to cover a larger area. However since this isn't working as well as we would like, if the ferrite tubes don't solve our issue I will be forced to mount the antenna pointing down from the ceiling and most likely need to add one or two more. Less than ideal but if it is necessary that is what we will do.

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Question - can someone explain why the 1900 MHZ is well blocked by grounded mesh while the 800 MHZ is barely affected? I understand frequencies and wavelengths but do not have enough experience to understand why the two frequencies are so different in how they respond to the shielding.

Thanks

 

[FvM]

The DB Pro is designed specifically to work with a dual band antenna FYI.

Obviously. I was just wondering if bad antenna VSWR might cause additional feedback problems. It's not the question if it can work gneerally with dual band antennas but if it works well with this specfic one.

I think it has been already meantioned by several contributors, but shield plates don't work like an optical mirror, or better, they work like a a mirror of µm size. The waves are difracted around it's edges. In addition, a small plate might even work as a resonator, and earth wires as an antenna.

Increasing the reflector/shield size may help.

 

[goyop]

OK final results

I did the following:
- added several ferrite tubes on both sides of the amp
- added one ferrite tube at each antenna
Got room for a minor bump of a couple db on the gain.

- Removed the ground wire that was attached to the 4 foot by 3 foot mesh behind the yagi
Got room for 3- 4 db increase in gain

- Detuned the yagi by 15 degrees (actually tried a variety of angles)
Got room for another 3 - 4 db of gain at the amp before feedback.

FINAL SUMMARY

Upon installation of the cell amp and exterior yagi plus indoor antenna we were seeing from -75/-80 db signal

After adding the mesh shield behind the yagi and turning the 1900 MHZ channel gain to the lowest setting we improved to -65/-70

After today's changes we are now at -57/-65 db signal

So from worst to best we achieved over 20 db improvement which will work just fine.


Thanks for all the input and help. If you have any other ideas I would enjoy hearing them.