Technical questions Radio

[Rahel]

Hi Guys,

I need your technical answers about the below points, thanks.

• If we have a separate antenna for Rx and Tx at your site, which one should be installed higher on the mast in a typical scenario? Why?
• Does 100W repeater/BS give more coverage compared to 50W repeater/BS? Please describe why?
• If the customer is going to install a site in the center of the targeted coverage area, which antenna you recommend them to use, Omni-Directional or Directional? What if the site is going to be in a corner of the targeted area?
• Does changing the antenna of portable radios with a higher gain antenna, affect the coverage of Downlink/Talk-Out? What about Uplink/Talk-In? Please describe the reason for your answer.

 

[chrismol1]

Is this some college course homework question? If so, answer Why. Please show work.

 

[Rahel]

Is this some college course homework question? If so, answer Why. Please show work.

I know the answers but I want to have different ideas about the way of thinking for these points

 

[mmckenna]

If we have a separate antenna for Rx and Tx at your site, which one should be installed higher on the mast in a typical scenario? Why?

Often the receive antenna goes at the higher level. When dealing with a lot of portable radios, getting a couple of watts from a small radio with an inefficient antenna needs all the help you can give it.

Does 100W repeater/BS give more coverage compared to 50W repeater/BS? Please describe why?

Slightly, but it depends on frequency, terrain, antenna height at both ends, etc.
Often new users will assume that 2x the power means 2x the range. Doesn't work that way. In a perfect situation, you need 4 times the power to double the range. But again, topology, antennas, etc. all come into play.
Where more power can help is in building penetration.
Inverse square law plays in here.

If the customer is going to install a site in the center of the targeted coverage area, which antenna you recommend them to use, Omni-Directional or Directional? What if the site is going to be in a corner of the targeted area?

Center of coverage area would -usually- benefit from an omnidirectional antenna in a perfect world, but not always. Lots of variables involved.
If the site was at the edge of the coverage area, then usually an antenna designed to send more power in the direction of the desired coverage would be beneficial, but again, lots of variables.

Does changing the antenna of portable radios with a higher gain antenna, affect the coverage of Downlink/Talk-Out? What about Uplink/Talk-In? Please describe the reason for your answer.

Slightly on both, but only a bit. The antenna is a small part of the equation. The metal body of the radio and capacitive coupling to the user will act as a counterpoise, so there's some variables there. But even that is dependent on frequency. Hard to get a good counterpoise on lower frequencies, easier on higher frequencies (UHF, 800MHz, etc).

If you get an A on your homework, I want partial credit...

Also, it's hard to simplify things like this. There are a lot of variables involved and it's not as simple as 2+2=4. This is why people get paid a lot of money to design radio systems.

 

[clbsquared]

The simple answer is......it's all about location, location, location. In a perfect world it would be as easy as 2+2=4. Without knowing the specifics of where this "system" is or will be, it's difficult to say which setup will provide the best coverage. Topography changes a lot of parameters.

 

[KevinC]

Since this doesn't appear to be Motorola specific I moved it to the Industry Discussion forum.

 

[prcguy]

Receive antenna always on top for reasons stated in post #3. Many towers have the top tier reserved only for receive antennas. 100w compared to 50w is a noticeable increase, not tremendous but if you are used to a 100w repeater and someone turns it down to 50w you may want the power back. This comes at a cost of tx to rx isolation with 100w requiring 3dB more isolation between tx and rx. If you have a marginal duplexer or tx/rx antenna spacing, going from 50w to 100w can push things over the edge and cause rx degradation. A 100w or more repeater requires very good components to play nice and if you want to add a rx preamp that requires even more care, isolation and filtering to make good use of it.

 

[wgbecks]

You have receive excellent advise from @mmckenna and others on this subject. I didn't seen any mention specifically in your post of any of the comments but wanted to state that if the application is full duplex (repeater) with separate transmit and receive antennas then placement of said antennas and electrical isolation is a very important aspect of any system planning and implementation.

The electrical isolation is necessary to prevent receiver desensitization that is much akin to trying to hear someone whisper across the room while another person is shouting loudly in your ear. Normally, duplexers having proper bandpass and band reject response between duplex ports is the most preferred solution as opposed to using separate antennas.

The degree of isolation depends upon many factors such as TX to RX frequency separation, receiver front end selectivity, IF selectivity, dynamic range, blocking and IM response in addition to transmitter noise and emission type employed for any given communications system.

 

[prcguy]

Even with a separate tx and rx antenna with vertical separation, you must have filters on the transmit and receive side to achieve enough isolation for a repeater to work. Vertical antenna separation might give you 10 to 25dB isolation but you need at least 70dB for a low power repeater with a good receiver and possibly 100dB for a 100 watt repeater with a lousy receiver. That extra isolation usually comes from cavity type band pass filters. Every repeater should have an isolator on the transmitter and its mandatory at most multi radio sites.

You have receive excellent advise from @mmckenna and others on this subject. I didn't seen any mention specifically in your post of any of the comments but wanted to state that if the application is full duplex (repeater) with separate transmit and receive antennas then placement of said antennas and electrical isolation is a very important aspect of any system planning and implementation.

The electrical isolation is necessary to prevent receiver desensitization that is much akin to trying to hear someone whisper across the room while another person is shouting loudly in your ear. Normally, duplexers having proper bandpass and band reject response between duplex ports is the most preferred solution as opposed to using separate antennas.

The degree of isolation depends upon many factors such as TX to RX frequency separation, receiver front end selectivity, IF selectivity, dynamic range, blocking and IM response in addition to transmitter noise and emission type employed for any given communications system.

 

[prcguy]

On the antenna question, you want the antenna to best serve the needed area, so if the repeater is in the center of the user area I would expect it to be omni. If its in the corner of the needed area you can choose an antenna with a directional pattern usually giving more gain. I've always gone with the highest gain antenna I can afford and this will pay off if handhelds are part of the system.

There are special circumstances where a high gain antenna might not be the best choice like a very high mountain top site where the users are mostly in nearby valleys close in to the repeater, otherwise lots of antenna gain and downtilt if the site is over 1,000ft high and antenna gain is over about 6dBd.

 

[freddaniel]

All good information thus far. Separate Tx - Rx antennas with multi-couplers are usually preferred because having more than 5 to 10 UHF repeaters with duplexers on a single tower is costly and difficult to manage interference. If the repeaters are owned and maintained by separate users, then it may be the only choice. However, if they are all owned or managed by the same company, then multi-coupling is preferred as interference is easier to control and the cost is a fraction of separate duplexers.
The primary downside to a master receive system is the single point of failure issue. This can also be overcome several ways, like having a spare receive antenna system that can be remotely switched, or using dual voted receivers on each repeater, each connected to a different receive antenna system, spaced at least 12 feet apart to overcome multi-path fading, especially on handhelds. Diversity receive can give handhelds at least 12-15 dB improvement. Some people also use circular polarization on master receiver antennas to reduce multipath fading.

See antenna isolation chart below to get an idea of what can be realized in isolation.

If you are in a community outside of a major city, then co-channel interference on the INPUT CHANNEL could be a problem. In that case, not using an omni antenna would be a good idea. By placing the repeaters on the side of town where the interference is coming from would be wise, as you could use panel antennas with no reception off the back of the antenna, thus no interference. If this is not possible, then you can put a notch in the pattern of a omni receive antenna by placing a V shaped reflector towards the interference.

Another system operator might use 1 to 3 voted satellite receivers on each repeater. Remember, you can make simple cheap analog radios work unbelievable with a great system. In some states you can get T1 lines cheap, or you can use microwave. If you were really driven, you would learn about the AllStar Linking hardware and how it might be applied to networking commercial radio, using voting & simulcast.

If you rented radios then the total system cost, including mobiles is on you. With digital radios there is no money to be made as the radios and system hardware is expensive, so there is no profit margin left for you. But with cheap analog radios and a smoken hot system, there might be some money left at the end of the month.

If money is no object, then Motorola has a DMR solution for you.

So many choices and options....

 

[902]

There are situations where receive is NOT on top. This is not the norm, but it's done for a very specific reason. That is typically when being on top of the tower places it closer to noise sources, such as FM or TV broadcast stations. We don't always have complete control of the sites we use, and when someone upsets an otherwise good site, other measures may be required - especially when there are no other sites that can cover the required area. An antenna may be used on the tower top, but usually with an isolator and harmonic filter. The receiving antenna will require vertical separation from the noise and will usually have directional characteristics... unless provisions have been made to place antennas in a manner that complements each other. The system may have asymmetrical coverage and may require voting receivers in the periphery to fill in for the loss of height. Additionally, preamplification may be used if the antenna is far enough away from the noise.

 

[902]

There are special circumstances where a high gain antenna might not be the best choice like a very high mountain top site where the users are mostly in nearby valleys close in to the repeater, otherwise lots of antenna gain and downtilt if the site is over 1,000ft high and antenna gain is over about 6dBd.

This was the case at the World Trade Center with the Port Authority's old analog UHF "Able" channel. The antenna was a 10 dBd SuperStationmaster on 1 WTC (the north building with the broadcast mast) and, especially with the high noisefloor, the site was deaf at street level and close-in. While troubleshooting coverage issues, I placed a unity gain DB-201 on 2 WTC (the building with the observation deck) square in the middle of the roof. This went to a voting receiver that used a house line to the comparator in 1 WTC. The repeater, an MSF5000, would key from the comparator or default to in-cabinet repeat absent the tone remote sequence. Close-in transmissions would vote to the unity antenna which was also horizontally separated from the much higher noise environment. Further-out transmissions would vote into the SuperStationmaster. Maybe. That site would have benefitted from 6 degree electrical downtilt.