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Thread: Antenna Questions

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    Default Antenna Questions

    Ok, I have a question or concerns. I have gathered some information and I'm posting this too the masses for some help and clarification on a conversation that I had with Sinclair and Comprod in the past few days.

    First, I know I've posted about repeater problems and what not. It has came to conclusion that we have antenna and coax issues, as well as, desense issue, not seen at the ground level but it is happening at the antenna level.

    Commscope makes our ava5-50 and they wanted me to check the ohn resistance at the ground level. I followed the steps for the ohm probes the outter ground of the hardline was zero and center pin was 19ohm with a mild wind and upto almost 40 in stronger winds. Commscope believes there is a bad connector at the antenna or on on the jumper to hardline. So, ok gotta get that fixed too.

    However, speaking with Comprod and discussing antenna options they are suggestion a 884-70 in bi-directional pattern or 872-70 facing our main coverage area and leaving everything behind the antenna as unity gain. The concurred with commscope as there is definitely something concerning with the ohm loss and increasing impedance with increased winds.

    Speaking with sinclair and discussing our problems and potential solutions. 1. move our antenna from 3ft off the tower to 6ft. and dropping our antenna from 305ft which is 567ft. AGL down to 190ft. and 460ft AGL or farther but woulding drop the AGL below 400ft. He stated that VHF likes to run along terrain instead of where it is now up in the air with no obstructions and according to his calculations the beam isn't coming close to terrain for 33mile away completely overshooting our area. He stated that the Commander 220-3BN at 8.1dbi where ok if we were in flat land and trying to talk out to 60mile mark or farther. He stated that the dbi needed to be about 5-6 and since our ERP is rated at 180watt and 280watt EIRP that we need to saturate our area and would bring the coverage area in closer to 20-25mile mark. He was explaining that down-tilt was not neccessary in this situation due to terrain. He suggestted SC281-HF3LDF and SC251HF3LDFD. He suggested this be placed 6ft from the tower, with the area with least coverage behind the antenna because it is going to null out. Sinclair also stated that we maybe trying to push too much over a mountainous area and needed 2 sites.

    A local radio tower/communications company is telling us this is all BS that we need to install a DB-224 at the current height and we will talk better than we ever have and they only require a 13in stand-off. Well, I'm not sold on that with as many as they have to replace here in mountainous WV and the ones they have installed have worked worse than what they have replaced.

    So what is y'alls take on this and thoughts on a solution.

    Thank You


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    Chief,

    We had a pretty in-depth discussion recently whereby you described the apparent lack of sensitivity of the fire repeater system since the lightning strike. I still maintain the most likely suspect is the antenna itself. You described a test done that proved good talkback range using a mobile radio operating simplex on the same antenna and line as used by the repeater and duplexer. You stated the repeater and duplexer were aligned and tests to insure they were well within specifications, that the antenna and line had been swept and found to have little or no return loss.

    What is really happening here is simple receiver desensitization! Not for the usual reasons of improper duplexer specifications or alignment, not due to a problem with the repeater’s transmitter or receiver, but rather due to what us old timers refer to as “Rusty Bolt” effect. This situation exists when something in the transmission path presents a non-linear response where it begins acting as a diode to rectify strong transmitter RF energy producing a broad spectrum of unwanted RF noise or interference. The resultant noise spectrum then passed down the receive branch of the duplex system to cause an apparent loss of sensitivity due to desensitization.

    This same electrical effect is better known to wireless providers as Passive Intermodulation (PIM) distortion and rears its ugly head when excited by multiple carriers (tones) output from transmitter combiners into a common transmission path and antenna system. Not only does PIM provide unwanted mixing products from combined transmitters, it can also include into the mix (no pun intended) the emissions from nearby antennas in close proximity as well.

    In recent years, wireless providers and consulting engineers utilize specialized test equipment that generates, combines, and amplifies specific pairs of frequencies for a given system bandwidth to test for the presence of predictable IM products that can easily be detected and measured if there is any non-linear response encountered in any of the transmission system components under test.

    On the subject of antennas, and height above ground, most of the advice you were given is totally uninformed BS! There is no finite “Rules of Thumb” that will work in all situations to dictate optimum height or placement for an antenna. To understand this, you have to realize that anything of a metallic nature appearing in the near-field of the antenna becomes a parasitic radiator, regardless of resonance.

    One has to take into consideration the physical dimensions of leg and face size, the orientation of its legs in relation to the surrounding geography, and the resultant pattern obtained from specific antenna type and placement about the tower. I’ve personally resorted to NEC modeling over the past twenty plus years to make best effort analysis and design of LMR antenna systems that yield the best possible pattern that encompasses the desired coverage area.

    The suggestion that you were given to lower the antenna to improve coverage ignores the real problem and has virtually no technical merit. It’s all about achieving an unobstructed path that takes into consideration land cover in addition to earth curvature. Can an antenna be to high? Yes, if it’s on a very high support structure or at location where a large disparity in elevations exist, and where mobile and portable users are operating in close proximity of the site or support structure within nulls in the radiation directed toward the immediate earth. The fix in these situations are use mechanical or electrical downtilt to direct greater energy toward the close in ground rather than to focus the lions share of signal at distant horizon.

    In closing, my phone offer stands, if you’d like to have your radio technician give me a call. I will gladly discuss theory and test methodology necessary to diagnose and correct this problem.

    MotoBill

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    Chief - Motobill told you in the antenna placement thread to perform the site noise test. You replied they did the 12db sinad. Two different things. If a simplex base station works, but a repeater does not, there's something wrong with your test, but it's giving you a useful result. If the duplexer has been checked out, and still doesn't work, there's something wrong with your test, but it's giving you a useful result. If they "swept" the line, and the antenna is good, they missed something in the interpretation of the results.

    You are trying to use VHF on a cell tower. That's your first mistake. You can easily expect 10db of noise from their gear, and because they are the incumbent on their own tower, you will get no relief. You won't be able to filter it out. It's wideband hash with spurs that hit individual channels worse than others, and it sounds like you found the jackpot in the Murphy's Law Lottery of Interference Probability. Congradulations.
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    Quote Originally Posted by Bill_G View Post
    You are trying to use VHF on a cell tower. That's your first mistake. You can easily expect 10db of noise from their gear...
    Can confirm, I personally experienced this trying to colocate a 2 meter amateur repeater on an AT&T GSM/LTE site. With the antenna at the same level as the panels the measured on-frequency noise was 20-30dB.

    The only other thing I can add is that the DB224 WAS an excellent antenna before Andrew bought Decibel Products and moved production down to Mexico. Overall construction quality and durability took a nosedive after that. I'd look at Telewave, Sinclair, Comprod, or another manufacturer for a folded dispole antenna.
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    Site noise is always a concern, especially on cell sites. Another contributing factor that rears its ugly head is RFI generated by strobe lights and LED marker lamps.

    However, in this case, the problem started following a lightning strike that took out the original repeater. Chief explained that the replacement repeater has been benchmarked and fond to meet specifications. He performed a test using a mobile radio operating in simplex and was able to communicate with distant mobile and portable stations.

    The problem only seems to show up when in full duplex, with the transmitter is keyed. I am very sure this is due to self generated desense being caused by something that was damaged by the lightning strike that now acts like a diode in the presence of transmitter level RF when applied to the system.

    You should be able to connect a spectrum analyzer to the receiver branch of the duplexer, observe the static noise floor, then key the transmitter and see what lights up the analyzer's display! I'd then connect a load resistor to the antenna port and repeat this test to verify that the duplexer is actually in alignment and that transmitter noise is nominal. Once that's been established, send the load resistor up the tower with the climber and have him isolate the antenna and recheck.

    I have always liked to have the tower climber carry a rubber mallet to tap on the antenna, jumper(s), fittings. The same applies when looking for intermittent scratching (desense) by taping on everything in the near-field of the antenna. I can't tell you how many times that poorly installed hardware have been found to cause intermittent noise by using this simple test. It just takes a little patience to explain the process while maintaining communications with the tower climber.

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    I would love to get my test equipment on this problem! It so painful to watch Chief801 go through this, with no competent techs around it seems.

    For those that do not know the history, this is related to the following threads.

    https://communications.support/threa...581#post113581

    https://communications.support/threa...ntenna-Spacing
    Radio Referenced...Those who think they know it all are very annoying to those of use who do.

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    I think the department is going to replace both antennas and feedlines. There's nothing else left that hasn't been replaced. So, now poses a question, Omni stick or exposed dipole? Sinclair still recommending SC251-HF3LDF, SC281-HF3LDF and for Dipole SD2352-SF2PALDF and mounting whichever one 7.5ft from tower based on Sinclair recommendations. Comprod still recommending 884-70.

    What are your thoughts, pros/cons and why?

    Thanks,

    Chief 801

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    Telewave ANT150D6-9 mounted at least one full wavelength from the tower. The spec sheet shows different radiation patterns that can be achieved with less than 1 full wavelength mounting from the tower: https://pronto-core-cdn.prontomarketing.com/185/wp-content/uploads/2016/07/TWDS-7045.pdf

    This antenna is not only the spiritual successor to the Decibel DB224, it's a much better product than the 224 ever was. The elements are anodized black and all of the connections are sealed with double-thickness adhesive shrink tube. I used its smaller 2-loop cousin on a 220 ham repeater install and I have nothing but good things to say about it.
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    Quote Originally Posted by motorola_otaku View Post
    Telewave ANT150D6-9 mounted at least one full wavelength from the tower. The spec sheet shows different radiation patterns that can be achieved with less than 1 full wavelength mounting from the tower: https://pronto-core-cdn.prontomarket.../TWDS-7045.pdf

    What do you mean wavelength? I mean how far is that?

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    Quote Originally Posted by Chief801 View Post
    What do you mean wavelength? I mean how far is that?
    https://www.everythingrf.com/rf-calc...-to-wavelength
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    Quote Originally Posted by motorola_otaku View Post
    So, if the TX is 156.105 it's saying 1.92m/6.3ft and RX is 159.525 is 1.9m/6.2ft thats one wavelength from the tower and that should be 1/4, 1/2, 3/8 of said difference away from the tower to reduce a null?

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    Quote Originally Posted by Chief801 View Post
    So, if the TX is 156.105 it's saying 1.92m/6.3ft and RX is 159.525 is 1.9m/6.2ft thats one wavelength from the tower and that should be 1/4, 1/2, 3/8 of said difference away from the tower to reduce a null?
    Since they don't make customer standoff lengths, six foot or longer will do.
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    Quote Originally Posted by Chief801 View Post
    That is an odd DTF sweep. It says either the antenna is over 45ft long, or about 80 ft prior to the antenna, there is a termination of some kind.
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    Quote Originally Posted by Chief801 View Post
    I've never seen a VHF antenna measure so flat. There is always a deep RL where it is most resonant.
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    Quote Originally Posted by Bill_G View Post
    Since they don't make customer standoff lengths, six foot or longer will do.
    Ugh. Autocorrect - custom lengths
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    I probably would have sweeped DTF at a wider frequency range than VHF only.
    Radio Referenced...Those who think they know it all are very annoying to those of use who do.

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    Quote Originally Posted by moetorola View Post
    I probably would have sweeped DTF at a wider frequency range than VHF only.
    I go 60M wide in all bands to get the highest granularity. 135-195 in VHF for example.

    So, what did you think of that haystack in the dtf plot?
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    Quote Originally Posted by moetorola View Post
    I probably would have sweeped DTF at a wider frequency range than VHF only.
    Our on site test and measurement Merlin agrees with you. He says the haystack points out the ambiguity of the Anritsu chirp method of sweeping, especially when looking at a wide band antenna in VHF. It's not a true TDR. But, you can circumvent that by using an extremely wide freq limit in DTF. So 100 to 900 in this case would be the best. So, the tech need to redo the test.
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    Hopefully, they are coming out again tomorrow pending snow.

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    Quote Originally Posted by Bill_G View Post
    I go 60M wide in all bands to get the highest granularity. 135-195 in VHF for example.

    So, what did you think of that haystack in the dtf plot?
    I agree, they need to rerun the sweep.
    That haystack seems typical of DTF sweep with narrow bandwidth. Cannot really see what going on with that narrow band setting.

    Like you already mentioned, I would sweep 100 ~900Mhz.

    Been awhile since I have used an Anristu, I use a bird sitehawk now. I guestion if it was even a calibrated sweep. Because of the two -- but maybe that means calibrated, not sure.
    On my sitehawk for 100~800mhz my DTF max is 500feet max and is what I would have ran the sweep at.
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    Chief,

    I'll run a NEC model to help in determination of antenna orientation and spacing if you can post the orientation of the tower legs, provide the diameter of the legs, and a measurement across the face of the tower from outside edge to outside edge. The ASR or site coordinates would be helpful as I can pickup some visual clues from the
    satellite photos to aid in constructing the model. Last, but most important, describe the coverage area(s) in relation to the tower site.

    MotoBill

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    Chief,

    I've used both the SD2352-SF2PALDF and the Comprod 884-70 and would not hesitat to reccomend the Comprod 884-70 in a heartbeat.

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    Motobill,

    38-38-13.5 N, 081-17-05.2 W
    ASR: 1002593 AMSL: 377m HAAT: 172.9m

    Tower Layout.jpg

    Antenna Orientation.jpg


    Terrain in the area is Mountainous, Rolling Hills, with farm land and heavily forrested areas area of coverage is 25km from antenna.

    Thanks

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    Homeland Security Radio Techs visited the site yesterday and here are the results of Antenna 1.

    Antenna I DTF Return Loss.jpgAntenna I DTF VSWR.jpgAntenna I Instertion Loss.jpgAntenna I Return Loss.jpgAntenna I VSWR.jpg
    Attached Images Attached Images