Using a parabolic dish for working the 70cm band - Am I missing something?

Question

I have read a lot of times in this forum that C-Band TV dishes are not suitable for operating 70cm. However:

• Suppose that we have a 2,4m dish. By assuming a global efficiency of a 60%, our total gain would be around 18,5dBi

• On the other hand, suppose that we want to achieve such gain with a common yagi antenna. Taking this calculator as an approximate reference, we would need a Yagi with more than 22 elements - which would be approximately 5 m long

In this case, I find worth going for a parabolic dish, since it can be shared between different bands (with different feeders) and is more mechanically consistent than a 5m long boom. Am I missing something? Why is then everyone so against dishes for UHF operation?

I know that the "optimal" way to obtain high gains at these frequencies would be using an antenna array. Nevertheless, a dish offers the possibility of working more than a single frequency band and takes up less space than a full 2x2 array.

Answer 1

First, the gain or efficiency of the dish will be lower than you estimate.

The outer quarter wave of the dish is substantially lost to diffraction. This is a 26% loss of area.

If it's centre fed, feed blockage will cost about 0.3 m² or another 10%, in practice double this because this is the best illuminated part of the dish.

Then uneven illumination and spillover will further reduce the efficiency by the usual 30-40%. For example, a yagi as a feed doesn't have symmetrical E and H plane patterns, so one will under-illuminate, the other will spill over.

So on a small dish, only 3.5 wavelengths across, I think efficiency will be well under 50%.

Finally the VSWR will be quite poor, with a yagi pointing almost straight at a reflector only 1 m away. This is where some of the feed blockage power goes.

The next step would be to simulate the whole dish and feed, try to optimise it and see what gain you can achieve.

By comparison, a yagi is quite a lightweight, low wind area construction. A 5 m long 70 cm yagi can be held in one hand, and it's balanced in the middle against gravity and wind. A 2.4 m dish, even if it's made of wires 5 cm apart, will be heavier and unbalanced and have a lot more wind resistance.

In the right regime dishes have a lot of advantages - you can use the same reflector at many frequencies, gain keeps rising with increased frequency, it's easy to make it circular or dual-polarised, wide-band feeds of 2:1 or more are possible. But the example you chose is on the wrong side of the line. For gains under 18 dBi, and narrow amateur bands, I think the yagi will be a simpler, lighter antenna.

Answer 2

How will you feed this dish? The feed antenna that comes on these antennas is not suitable for 70cm. The ideal feed antenna illuminates the dish and nothing else. To achieve such an illumination you'd want something like a Yagi or a horn antenna.

After building such an antenna for 70cm and attempting to mount it on the dish, I think you'll quickly realize the dish appears ridiculously small in comparison. The reason you don't see tiny dishes on large feed antennas is they are very inefficient. Diffraction around the edges of the dish, which is not very large compared to wavelength, will degrade efficiency. Furthermore, the dish is most effective when the feed antenna is a single point at the focal point: when the feed antenna is relatively large this is less true, leading to degraded focus and efficiency.

In the end, you'll find adding more elements to a Yagi is more effective than adding a dish, unless you plan to add a much larger dish. For these reasons, parabolic reflectors usually only make sense when they are large compared to wavelength, which a typical rooftop dish is not on 70 cm.