Antenna Prblem - RF Cafe Forums

Post subject: Antenna Prblem

Posted: Thu May 18, 2006 5:02 am

Hello...

I am RF Engineer and working with one of the biggest broadcasting company for its DTH services. Well We are having the biggest Earth station as an private broadcaster. Well I am facing one problem regarding Antenna. We are using one 9.4m (A)Andrew Antenna for continues TX and one 9.4m (B) same antenna as redundent. we have 7 transponders on which our services are running. The antenna is Linearly polarized. The problem is when we switch our services from 9.4m antenna A to antenna B all the transponders work properly except one. We also have one 4.6m antenna for redundancy purpose. When we switch from 9.4m A to 4.6m all transponders and entire service work properly. We conclude that there is some problem in our 9.4m B antenna. We checked all parameters like polarisation, feed status, cross polarisation etc but still not able to locate the problem. I dont know why only single transponder get effected. If any one can suggest some remedies or some solution I will be highly obliged. To know more about the problem one can contect me on my mail id :mayankkhanna007@yahoo.com.

Regards

Mayank khanna
RF Engineer

Post subject:

Posted: Thu May 18, 2006 8:07 am

Greetings Mayank Kumar Khanna:

I must agree that it does not make sense that there would be any reason from an RF standpoint that only one transceiver would shut down when switching antennas. Is it possible that an RF switch is not getting the proper command from the controller or that the transceiver itself is not being commanded? It could be a fairly simple software issue.

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- Kirt Blattenberger
RF Cafe Progenitor & Webmaster

Post subject: RE:Antenna Problem

Posted: Thu May 18, 2006 11:38 am

Dear Kirt Blattenberger,

Thanks for the reply. I really appriciate your quick response.First of all I would like to give brief idea about our MCPC carrier that is we have four vertical carrier combine together and are amlified by vertical HPA and three horizontal carrier combine together and amlified by single horizontal HPA. Though before antenna system we have switching unit whoes software is design by APOLLO. But I dont understand why only one vertical trans transponder get effected why not the entire vertical chain get effected if there is some problem in switching software coz the entire vertical carrier is passed through one waveguides to the antenna. similarly for Horizontal carrier also. At a distance of say 100m there is one mobile tower but it is not in path of the antenna. Is it possible that there is some interference due to mobile antenna as most of the mobile communication took place at vertical polarity and our vertical transponder is getting effected. But mobile communication occurs at very low frequency and we are transmitting on KU-Band. If you can answer or give some specific solution for the problem or if you can provide me with some stepwise procedure to check the fault then I will be greatfull to you.Plz mention your mail id so that i can cotact you in future for any assistance.

Thank you.

Regards

Mayank kumar Khanna

Post subject: Transponder Frequencies?

Posted: Fri May 19, 2006 3:06 pm

Is it a fair assumption to say that the transponders are at different frequencies?

If so, is the problematic transponder at one of the edges of the vertical bandwidth (i.e. either lowest or highest frequency)? Are you shooting the link at a low elevation angle? This may help locate the source of the problem.

Can you tell if you can capture some external signal at (or near) the "problematic" transponder frequency? In any of the Tx antennas or Rx antenna?

Post subject: RE:Antenna Problem

Posted: Sat May 20, 2006 2:08 am

Respected Sir,

Thanks for the reply. Well yes we are transmitting on seven different transponders on same satellite. As such the elevation angle is not at all low. The elevation angle is 55 degrees. As such we are having lot of RX antenna tuned to different satellites at different frequencies and similarly we also have 7 TX antenna ranging from 4.6m to 9.4m. But leaving these three antenna rest are tuned on C-band. I am still not finding any such specific solution for it. As far as RX frequency is concern it cannot down the TX frequency no matter it is at same RX frequency of same polarity.Plz specify some more reason for the same.

Regards

Mayank

Post subject:

Posted: Sat May 20, 2006 4:25 am

Maybe I'm just dense but bear with me.

You have one Tx antenna (A) which shares 4 vertically polarized transponders. These antenna is fed from one HPA which in turn is fed by the combinations of all four transponder frequencies.

You have a second Tx antenna (B) which serves as a backup (or hot-standby) to the first antenna. When switched properly, it is fed from the same HPA as the first one (i.e. the feed is switched from antenna A to antenna B).

The first antenna (A) allows all transponders to operate properly, while the second antenna (B) allows only three of the four to operate properly.

If this is a good summary of the problem, then the problem most likely lies in the hardware (switch, waveguide feed to second antenna, or second antenna itself). If the transponder that's showing the problem is at either the lowest or highest frequency (of the four), then there could be something where one of the above mentioned components is not allowing this frequency component to pass (e.g. in the waveguide, the lowest frequency could suffer if the wrong dimmension is used for the waveguide). Another thing that could be happening is that some outside interferer is affecting the one antenna and not the other (are you operating on licensed frequencies? - I would think so but it never hurts to ask).

So, if any of this could apply to the problem then the original two questions are still important:

1) Is the problematic transponder at one of the frequency edges of the vertical bandwidth (i.e. either lowest or highest frequency)?

2) Can you capture some external signal at (or near) the "problematic" transponder frequency? In any of the Tx antennas or Rx antenna? Meaning, if you're tuned to the frequency of the transponder in question, do you see (in a Spetrum Analyzer) some signal other than the desired.

Hope this helps.

Post subject: RE:Antenna Problem

Posted: Sat May 20, 2006 8:32 am

Respected Sir,

I really appriciate your quick response.You have judged correctly. We have overall 7 Transponders in which 4 are vertical TX and 3 horizontal TX transmittingfrom a single 9.4m antenna A. I would like to tell you one more thing that One vertical and one horizontal TX has same TX frequency which is the lowest among all and the vertical one is getting effected. We were earlier thinking the case of crosspol but then since all the transponders are working fine on antenna A so this case is also not valid( I think so). What we are doing is again taking plots of antenna B( problematic antenna) and then check, may be antenna is having problem. After checking the antenna if the plots are OK then i will check as per the solution you have provide to us. In case you find some more reason plz do mail me. Yes we are operating on licensed freq.

Regards

Mayank

Post subject:

Posted: Sat May 20, 2006 3:09 pm

I think you're on the correct path. Since you have two Transponders on the same frequency (one vertical and one horizontal), cross-pol on the "bad" antenna (B) would be the first thing to try.

Post subject: RE:Antenna Problem

Posted: Mon May 22, 2006 10:18 am

Respected Sir,

I would like to ask the reason about the point you made that: Is the problematic transponder at one of the frequency edges of the vertical bandwidth (i.e. either lowest or highest frequency)? . The Transponder which is getting effected is at the lowest trans frequency among the seven transponder of ours.As a whole it is the second vertical transponder on the satelliet.First vertical Transponder has been taken by government DTH service provider. But could you plz explain me why being a lowest frequency it is getting effected. Does this also have some sort of impact. Plz let me know te answer in detail.Waiting for your reply.

Thanking you.

Regards

Mayank.

Post subject:

Posted: Mon May 22, 2006 3:12 pm

If you have a problem with only one transponder frequency and it would appear to be something related to the hardware, my intuition would say that it would be located at one of the band edges. I explain myself, if the problem is in the antenna (or the antenna feed) it would be hard to imagine (though not impossible) that it would be in mid-band (since you're only experiencing it on one frequency). You would think that if it the problem occurred in mid-band more than one transponder frequencies would be affected. Thus, my comment that I would expect the problem frequency to be on the band edges.

Now, it seems you have one antenna setup that works just fine and a "similar" one that gives you this problem. You also mention that same frequency is shared by another transponder with horizontal polarization. So then I would have the following questions:

1. Is this the first time you have used this second antenna?
2. Have you measured the cross-polarization on the "good" and the "not-so-good" antenna. Do they match?
3. If the measured cross-pol for both antennas is similar, could you measure the incoming "trouble" signal at both antennas? How does it compare?
4. How does the antenna installation differ between the two antennas? Are they horizontal to each other, vertical? There could be some outside interference affecting one antenna and not the other based on antenna location - the previous step should shed some light on that

You have probably done some (if not all) of this already. Just pointing the "easy" things first.

Post subject:

Posted: Wed May 24, 2006 8:10 pm

Mayank khanna

I saw from one of your below replies, I copy here "I would like to tell you one more thing that One vertical and one horizontal TX has same TX frequency which is the lowest among all and the vertical one is getting effected" So I understand the problem is the MCPC carrier on that vertical uplinking transponder.

Let's call this transponder as XP V1.

If I were you, I will do the followings.
1) Validate from Spec An plot of vertial HPA Monitor port that you are transmitting a desired MCPC carrier on XP V1 by tunnning whatever center frequency of XP V1. ; if you don't see the MCPC carrier on V-HPA, then maybe some cabling is missing from your KU upconverter.
If you see the MCPC carrier on XP V1 from HPA sample port, then go to 2.

2) Validate the proper cross-pole isolation of Antenna B.
IMHO(in my humble opinion), Antenna B doesn't seem to have enough cross-pole isolation between V and H polarities.

The best way of doing this in your circumstnace ( assuming you have a good working antenna A) would be to transmit a CW ( pure carrier) on vertical ( could also be horizontal) uplinking polarity from Antenna B and monitor the CW downlink from Antenna A's vertical and horizontal polarities. As you may know, V- uplink should be monitored in H-downlink as linear satellite coverts polarity of up and down links. It means you have to see a bigger size of CW carrier in H downlink of Antenna A if you are uplinking CW in V pole from Antenna B. You will need to compare H and V downlink fo antenna A in order to determine a valid isolation. If considering the size of antenna, 9M, I would recommend at least 27~30 dB of X-pole isolation, meaning CW C/N of co-pole(H) downlink should be 27~30 dB higher than that of X-pole( V) downlink.



HTH


Jongsoo Kim

Post subject:

Posted: Wed May 24, 2006 8:36 pm

I like tp post my reason that I think the problem is a poor isolation of polarity.

The porblem occurs vertical XPNDR whose frequency is also in horizontal XPNDR. If the isolation of linear polarity is poor, all MCPC carriers will be transmitted to both polarity. Depending on degree of off-tune, one of polarity will receive more energy than the other. However, MCPC carriers on 5 XPNDR would be OK( unnoticeable) because they don't have frequency overlap: well it is not ok as it will knock down whatever carriers in 5 XPNDR on the other polarity using same frequency. MCPC carriers on 2 XPNDR that have same frequency but different polarity to each other would be immediately noticeable by this polarity leaking problem becasue they will directly affect each other. In this case, a MCPC carrier on H up/V down XPNDR survived while the other carrier on V up/H down was down; which link will survive would be a matter of how much link extra margin exists.


Jongsoo Kim

Post subject: RE:Antenna Problem

Posted: Fri May 26, 2006 11:29 pm

Hello Jongsoo Kim,

I am greatfull to you for suggesting me with lots of options.I am going through the procedure and hopefully we will solve the problem. I will revert you as we solve the problem and let you know the cause.

Regards

Mayank