Biquad on Dish Question

Ok, let's play with the numbers. First, with a reflector type antenna, the gain of the biquad feed is not important. It's sole purpose is to "illuminate" the reflector. If the beamwidth of the feed is too wide, you lose gain in transmit. If the beamwidth of the feed is too narrow, you lose gain in both receive and xmit because you're not using the full area of the reflector. See:

When dealing with elliptical reflectors, that are designed to handle multiple feeds (for multiple satellites), you don't get the benefits of the entire dish. The offset feed also means the effective cross sectional area is somewhat less than the physical area. I'm too lazy to work out the exact number, but my guess(tm) is that the DirecTV dish has an effective circular diameter of about 60 cm. In other words, the bulk of the reflector is not used.

The maximum gain of a dish antenna can be calculated. Assuming you did everything perfectly, and the feed was properly matched to the dish f/D ratio, then: Maximum gain for a 60 cm meter diameter dish: gain = 9.87 * Dia^2 / wavelength^2 * (feed efficiency) gain = 9.87 * 600mm^2 / 125mm^2 * 0.4 gain = 91 dBi = 10 log(91) = 19.5dBi The 40% efficiency (that's the 0.4) is probably optimistic for your home made feed. Again, the numbers assume that everything is lossless, perfectly matched, built correctly, and properly designed.

Your 10dB difference is gain is very close to what I would expect (i.e. calculate). The typical biquad has 8-11dBi gain. See:

So if I give the biquad a 9dBi gain and the dish perhaps 18dBi of theoretical gain, then an 9dB difference is about what you would see. Close enough to your 10dB difference.

Some stuff on dishes and feeds from my bookmark pile:

MFJ means "Might Fine Junk". They also no longer sell or support the MFJ-1800 antenna. The link:

is dead. My guess(tm) is about 13dBi gain at best.

Ummmm..... yeah, that's an understatement. Actually, it's not difficult to make the measurement. The problem is finding a place to do the testing that isn't full of reflections and interference. I've found the deep dark forest to be fairly good as the trees absorb all the reflections. The only problem is that I have to elevate the test antennas well above the roadway to prevent ground bounces.

I have a bad attitude about yagis. They have their place but are a PITA to build, maintain, install, and design. I much prefer a panel antenna which is much simpler, easier to mount, waterproof, wider bandwidth, and only slightly more dollars. How about 13dBi gain for $36?

Thanks for all that info Jeff,

I'll take some time to digest it, especially chapter 4. Also thanks for for the link to the panel antenna. I think I'll purchase that just to compare to the yagi. Oh, I find the yagi link does work.

Two more questions. Regarding you formula;

Where does the 9.87 come from?

At my boat location, I find I need to have a signal strength of -76dbm to log on. If any weaker it won't work. At home I logged on with -90dbm with good speed. One big difference between locations is this, on the boat I have 10 to 14 signals that I can see, at home it's one signal.

Why do I need the stronger signal on the boat. (interference?) Thanks, Mike

So much for my bookmarks. Note that the MFJ-1800 is NOT listed on the antennas page:

Groan...

See formula at:

It's Pi squared.

Welcome to the joy of interference. You might be able to improve things at your end using a highly directional antenna. However, the interference will still be present at the access point end of the link, resulting in no real improvement. Also, high gain antennas have narrow beamwidths, which causes problems as the vessel rolls in the waves.

Also, see:

Probably. The big question is interference at which end of the link? If it's at your end, there's plenty you can do with antennas. If it's at the access point end, there's not much you can do.

Thanks Jeff, Sorry, I missed the Pi^2 on that page, I looked several places and couldn't find it. I'll study and try to leave you alone,,, for awhile! Thanks again, Mike

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I have some friends who tinkered around a lot trying to get a used sat TV dish to work. They had simliar results as the OP. I think they finally opted for a QUagi antenna consisting of quad driven element reflector and one director and several yagi type directors. They also experimented with some of the large TVRO antennas and the gain on these was awsome.

Jimmie

I've done it successfully. This is an early version:

The only tricks are to make sure the feed matches the f/D ratio of the dish, and to *NOT* use dishes made from multiple LNB's.

Reminder.... there is a legal limit to EIRP.

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I think I found the same antenna a little cheaper, but to late for me.

$31.00

Yeah, the photos look identical. There's also:

along with a mess of others in the same price range and gain spec.

Check out the data sheet on this page; (The 13dbi is the one we're discusing)

shows the 13dbi, 16dbi, and 19dbi Beampatterns. The 19dbi has a lot of sidelobes! Btw, I put my biquad on the 18" by 22" dish. The best I could get is 4 db over the MFJ Yagi. Might be able to tweak a little more out of it. Mike