Multiple feeds

Discussions about low-noise blocks, waveguides, horns and other components used at the prime focus of your reflector dish.
Post Reply
chrs75
Posts: 9
Joined: Fri May 29, 2015 2:40 pm

Multiple feeds

Post by chrs75 » Sun Sep 23, 2018 3:18 pm

Just wondering, I have a 10ft. mesh Cband dish and an extra C band and Ku band lnb lying around. Is it worth it to add a Ku lnb in my set up. Not sure if. I would get good ku signal.

Thanks for your input.

Chris

User avatar
tek2000
TVRO Salesman / Sponsor
Posts: 218
Joined: Fri Jun 20, 2014 4:38 am
Location: Canada
Contact:

Re: Multiple feeds

Post by tek2000 » Mon Sep 24, 2018 5:31 am

Hi chrs75,

Yes, it can be done and is well worth the effort. You will have to drill an opening in your scalar ring to let the ku feed through. If you have a c/ku LNBF, unscrew the ku LNBF and use that because it works really well. You will need a 2" drill bit to make the hole in the scalar. Drive your dish to an active ku-band satellite (e.g. 97W, 103W or 125W) and adjust the skew and phase center of the off-center feed until the signal is peaked. You must drill the hole *exactly* along the Clarke Belt (satellite arc) or it won't work well. To understand what I mean, have a look at

8.5: Multi-Focus and Off-Axis Feeds


viewtopic.php?f=146&t=1252

multi_focus_feed_system.JPG
Owner
http://www.tek2000.com
#1 Supplier of TVRO Satellite Equipment in North America

(I am not on this forum often and don't check forum PMs, so please email me for faster service)

MrVideo
Posts: 15
Joined: Wed Feb 28, 2018 3:35 pm

Re: Multiple feeds

Post by MrVideo » Fri Sep 28, 2018 12:19 pm

chrs75 wrote:
Sun Sep 23, 2018 3:18 pm
Just wondering, I have a 10ft. mesh Cband dish and an extra C band and Ku band lnb lying around.
Is it C-band only mesh dish, or a dish that is cut for Ku? If the mesh holes are too large, the Ku signal will not reflect off the mesh, instead going thru the holes that are only cut for C-band.

User avatar
tek2000
TVRO Salesman / Sponsor
Posts: 218
Joined: Fri Jun 20, 2014 4:38 am
Location: Canada
Contact:

Re: Multiple feeds

Post by tek2000 » Fri Sep 28, 2018 6:48 pm

MrVideo wrote:
Fri Sep 28, 2018 12:19 pm
chrs75 wrote:
Sun Sep 23, 2018 3:18 pm
Just wondering, I have a 10ft. mesh Cband dish and an extra C band and Ku band lnb lying around.
Is it C-band only mesh dish, or a dish that is cut for Ku? If the mesh holes are too large, the Ku signal will not reflect off the mesh, instead going thru the holes that are only cut for C-band.
A good first order approximation for analyzing the energy reflected from a thick mesh surface is to think of the holes as "tiny" circular waveguides. As is well known, above a certain cutoff frequency, RF energy will not propagate through the waveguide. The cutoff frequency (GHz) for such a waveguide is given by:

Code: Select all

fc = 17.59/d
where d is the diameter of the hole (in cm). For the tek2000 mesh antennas, the diameter of the mesh holes is about 0.2 cm. So fc = 87.95 GHz. Even for d = 1.0 cm, fc = 17.59 GHz.

Some people think RF waves are like water and the dish is like a bird bath. So if there are holes the dish, the water must drip through the holes. :rotflmao: Sorry, but it doesn`t work that way. There is so much confusion out there between the performance of a mesh vs. solid antennas that it`s pathetic. The main reasons for the poor performance of a c-band dish in the ku-band is due to:

1. Poor parabolic surface accuracy;

2. Poor feed illumination


Assuming you have a decent quality dish, reason # 2 is almost always the prime culprit.
Owner
http://www.tek2000.com
#1 Supplier of TVRO Satellite Equipment in North America

(I am not on this forum often and don't check forum PMs, so please email me for faster service)

MrVideo
Posts: 15
Joined: Wed Feb 28, 2018 3:35 pm

Re: Multiple feeds

Post by MrVideo » Fri Sep 28, 2018 9:23 pm

tek2000 wrote:
Fri Sep 28, 2018 6:48 pm
Sorry, but it doesn`t work that way. There is so much confusion out there between the performance of a mesh vs. solid antennas that it`s pathetic.
Sorry, but I'll have to disagree. I have been dealing with a person who has been in the satellite business for years. He work for a satellite dish manufacturer for years (who is now out of the business), also distributing and setting up C-band dishes. For Ku reception, the smaller holed mesh was required.

We'll just have to agree to disagree. As for the OP, it would be nice to know if he has the larger, or smaller, mesh and if adding Ku works, or not.

As for adding Ku, why not get a C/Ku feed horn? That is what I have on my 12ft. I very seldom used the Ku after I put it in and certainly don't now. I get very little of my programming from my dishes. I use them for wild feeds. I can't stand ads, bugs and low bitrates. But, that is me.

User avatar
fatso
TVRO Guru
Posts: 797
Joined: Wed Aug 13, 2014 7:46 pm

Re: Multiple feeds

Post by fatso » Sat Sep 29, 2018 1:59 am

I would do a ball point pen test on the mesh. If the pen tip doesn't go thru then you should get excellent results for ku. :bigsmile I've seen mesh holes (and perforated aluminum holes) as large as 1/4" and ku works fine. But there is more to it as tek200 said. The paraclipse was considered top of the line for c-band reception back in the day, but nobody bragged about the ku reception, even through the holes were pretty small too. Go figure. :noidea:
12ft Mesh Dish
C-Band Enthusiast since 1983

User avatar
tek2000
TVRO Salesman / Sponsor
Posts: 218
Joined: Fri Jun 20, 2014 4:38 am
Location: Canada
Contact:

Re: Multiple feeds

Post by tek2000 » Tue Oct 02, 2018 5:04 am

MrVideo wrote:
Fri Sep 28, 2018 9:23 pm
Sorry, but I'll have to disagree. I have been dealing with a person who has been in the satellite business for years. He work for a satellite dish manufacturer for years (who is now out of the business), also distributing and setting up C-band dishes. For Ku reception, the smaller holed mesh was required.
The best analogy I can give you is that of a diode from electronics. If the diode bias voltage is above the diode threshold voltage, the diode will conduct. If it is below the threshold voltage, no current is conducted. Around the threshold voltage is "no-mans" land.

With mesh antennas, there is a threshold hole size. If the hole is too large, hardly any RF energy gets reflected. Below the threshold hole size, all the RF energy will get reflected. Whether your mesh holes are 1mm, 2mm, 3mm, 4mm or 5mm, your antenna will reflect roughly the same amount of energy.

I don't think anyone back in the 80s proved this idea rigorously, because personal computers were only just becoming available. I haven't seen a rigorous analysis in any antenna engineering books, either. If someone wants to do the analysis now and show us how the antenna gain varies when you have perforated surface with varying hole diameters, you will WIN a FREE 10ft Mesh Dish from Tek2000!
Owner
http://www.tek2000.com
#1 Supplier of TVRO Satellite Equipment in North America

(I am not on this forum often and don't check forum PMs, so please email me for faster service)

Post Reply

Return to “Feed Systems”