# Is there any point in using a large dish on RX?

• Then I assume that in the case of DATV, the noise floor of the transponder must not be as easy to see as it is on the NB transponder.

Because if you can clearly see both the level of the beacon AND the level of noise on the screen of your receiver, using a small dish, then using a 30-meter one would have the same effect as running the received signal through a (linear) amplifier. The sihnal/noise ratio would not improve.

This is the very point of this thread. Why should a larger dish improve the signal/noise ratio of a received signal from the Oscar 100 transponder? This signal is broadcasted by the sat with a given s/n ratio, which cannot be improved lest breaching some thermodynamics law.

• Well, that reasoning applies after you have a certain magnitude of the transponder noise.

When the tpx noise was around 10 dB more (the first few months) the perceived SNR (with the same dish) was higher.

Imagine the following experiment:

Suppose you have the tpx noise floor with +10 dB over your system noise and the tpx relays a signal which is 3 dB above the tpx noise floor.

Then the overall tpx gain/power (whatever) is reduced with 10 dB so that the tpx noise is equal to your system noise (and the 3 dB signal is still being relayed).

Having the tpx equally strong as your system noise, the whole noise figure (or SNR) degrades by 3 dB (now you have twice as much noise, because the two noise sources are independent, they can be added stochastically) which also makes the 3 dB signal vanish (into the noise).

• That's right. It doesn't matter which transponder, you still gain with a larger dish, you just gain less and less. At some point the extra gain is not worthwhile so the optimum shifts. With narrow band, it is probably not worth using more than 1.2m.

You could have 50% contribution to the nose on uplink and downlink, this is balanced 50:50 and 3dB down on the best possible. On the NB transponder that's a really small dish, about 60cm or maybe less with a good LNB.

Mike

• Official Post

I was planning to carve a lens to be fitted in front of my LNB, to improve efficiency. The material I was planning to use is wax. Candle wax. Wax being a good dielectric, losses should be low, I hoped.

High frequency dielectric is characterized by both dielectric constant (the famous epsilon) and the dissipation factor or dielectric loss tangent (often quoted as tan-delta).

I would be very surprised to see a measurement for candle wax as its composition is anything but well defined. At best you could try to find it out for paraffin, but given its low melting point of 37degC, I would not recommend it for use in a feed.

Teflon is easily available, is well documented and has excellent properties at high frequencies. Try to carve a lens based on Willi's drawing.

https://rfantennas.wordpress.c…-dualband-feed-vergleich/

I just did so and upgraded my home setup to a POTY type with a teflon lens. No drop in RX performance compared to LNB-only setup (transponder noise still ca. 4dB above thermal noise on my 88cm dish) but I have now a TX feed in focus (wasn't the case up to now). More news next weekend at the UKW-Tagung in Weinheim.

• I made a comparison between POTY with and without a Rocket lens (Venton).

Mounting in front of a 1.2m offset dish, 0.66 F / D

Measured with Siglent Analyzer and 20x averaging.

LNB is a modified Venton Rocket EXL-S with ext. LO.

(due to the averaging, the lower beacon is only half as strong as the upper one)

73 de dd9fj / Michael

## Images

• dd9fj Note that when you place a (dielectric) lens on front of the wave guide, the phase point shifts (towards the dish), so you may need to re-tweak the contraption for optimal SNR/sig strenght.

• I know, but by the construction of the support I am limited in the ways.

The POTY is pushed as far as possible in the focus,

for further experiments, I would have to change the mount or the reflektor..

dd9fj / Michael

• dd9fj Michael, that is the reason I posted .. all should keep an eye on the distance between the boom and the lnb mounting to keep the reflektor as big as possible. on my dishes it works perfect.. Pictures you can find here in the forum..

DD0KP - Heiner - Waldkirch - JN38XC

• dd9fj When you twist the contraption 45 degrees, is it possible to increase the dish-lens distance somewhat?

• It would be easy to modify the 40mm to 22mm adapter I see there to allow the feed to be 6mm higher and that might be enough. It doesn't have to be exactly centred but you may need to adjust the elevation a little to compensate.

• dd9fj how did you attache the Rocket lens (Venton) to the potty antenna? What kind of glue? On my side diameters are closed but different and it doesn't fit well. I need to stick the internal part of the rocket into the cupper tube... That rocket is made of 2 different parts, is it important to have those tow parts totally nested into each others? Thanks.

• dd9fj how did you attache the Rocket lens (Venton) to the potty antenna? What kind of glue? On my side diameters are closed but different and it doesn't fit well. I need to stick the internal part of the rocket into the cupper tube... That rocket is made of 2 different parts, is it important to have those tow parts totally nested into each others? Thanks.

I used a small bit of electrical tape to hold it in position.

Mike

• Hallo,

I milled a little brass ring which is soldered to the copper tube. The ring fixes the lense.
Brass can be found in the plumbers junk box.
From my point of view, the diagrams from DD9FJ above show an effect I could see also. The increasing of the gain is less then the reduction of the background noise. The diagram gets smaller when using the lense and there is less noise receiving beside the dish. As a result the SNR is higher.
This can be seen more clearly, receiving the NB with a wide spectrum display.
So, the lense works as expected.

73s
Andreas