420~450 MHz feed for 3.2m dish?

[Coto]

Hi all,

 

My friend's got a 3.2-meter dish (very smooth = proper for higher frequencies too) and I wanted to try Pulsar observing with it. Does anyone know of a reliable feed for the 420~450 MHz band to use my G8FEK LNA with and possibly achieve some nice results? I haven't been able to find a lot of designs online for such a low frequency (maybe log-periodic ones, but these are wideband and are probably not very efficient at the center frequency I want them to be best at).

 

Anyone got any ideas?

[henry b]

Circular or linear polarization?  or  both !      connector type,   temperture ? mounting ? Gain and beamwidth required ?  yes i have ideas.......... Henry b

[BiggarDigger]

At that band, a 3.2m dish is probably the smallest practical size and even then, depending on the mount and base, you may be in danger of giving yourself additional engineering challenges that could be removed by going to stacked and bayed yagis.

A 3.2m dish should yield a forward gain of around 20-22dBi at those frequencies, which is probably a couple of dB less than an array of four 15 element yagis stacked and bayed, or a multi-element phased array.  Given the size of the yagi, the array is pretty small, lightweight and has very little wind loading, so a simple ground pole with az-el control should match the dish.  Better still, if you can just arrange elevation control, you can run drift scans across your chosen target, so the mounting arrangement becomes considerably less complex.

However, don't underestimate the stacking and baying phasing and accuracy to achieve an optimum solution and if you want to go for circular polarisation, then such a solution is not viable.

If you decide to go with the dish and it's already mounted and driven, a small 2 or 3 element yagi, centred on the 432MHz amateur band may suffice, mounted on a suitable A-frame.  You'll need to get the spacing correct from the front of the dish to the driven element and a worm drive may help tune that for optimum gain/bandwidth.  The smoothness or finish of the dish is surprisingly tolerant to surface defects and abnormalities, especially at such low frequencies, so if you need to attach additional brackets for the mounting frame, these should make little difference to the performance.

Circular polarisation should give you good coverage and some years ago there used to be amateur radio satellite antennas available in that band.  I see no reason why these shouldn't still be available from various suppliers.  Try to use the lowest loss co-ax feed as you can afford, even with the LNA mounted at mast head.

Good luck!

Richard

[henry b]

Assuming the dish is well mounted and motorised  consider a wideband feed concept for other frequency operation, the feed could be step motor controlled for phase changes, feed changes should be simple!  the dish front to back ratio will be an advantage as you increase your operating frequency. Substrate PCB designs feed concepts will deliver interesting results.                 Henry b.