Inside the RF HamDesign feed

[VK3ALB]

Hi All,

Have you ever wondered what's inside the RF HamDesign feed? After you've seen inside perhaps it might interest you to read a little about the design here.

[VK3PY]

Yikes, Lou! How on earth was the placement of the 23cm loop chosen, I wonder? Also, that reflector "disc" looks like it was cut very approximately. Not the latter would make any significant difference to the feed's performance, but hardly what you'd expect for a pretty pricey item. Owners of such feeds must now be wondering what's inside theirs!

For anyone interested in constructing their own microwave loop feeds, here's a dual-band ring feed for 2.4/3.4 GHz that I constructed some years ago. It employs a common connector for the two bands: viewtopic.php?f=45&t=11410&start=260#p50822

Chas
VK3PY

[VK3ALB]

Hi Chas,

I suspect that the 1296 loop was probably distorted as the result of overzealous tightening of the coax connector or perhaps twisting of the coax during dish rotation.

Yes, at first view inside my reaction was also yikes but I'm a "feed half full kinda guy" and would like to think that this view gives us incentive to have a shot at building our own version.

Questions I have now are;

• How critical is the shape, position of the loop?

• Where is the phase centre and does it really matter?

• The reflector is only grounded at 1296 and 10GHz. Deliberate?

• There's no metal plate around the circumference of the feed. Would it benefit from the addition of one? (see other document)

• You told me your feed was a little tricky to get right on just two bands. I wonder how these are massed produced for the ham market? There might be something to learn there?

[VK4GHZ]

I was a little disappointed with the performance of my RF Hamdesign tri-band (3.4/5.7/10 GHz) ring feed, but I guess they should be considered as being a compromise (with questionable performance) vs optimised single band feeds.

Fun to play with, but as Chas points out, they are not particularly cheap either.
I no longer needed it when I bailed out of the near-zero-activity-in-VK4 3.4 GHz band .

Interesting article, as I wasn't prepared to 'crack the seal' on mine and have a look inside.

[VK3PY]

Lou,

A reasonable explanation for the distorted 23cm loop.

The phase centre is band-dependent on this kind of feed. The accuracy of placement relative to the dish focus is most critical for the highest frequency, so that's what you aim for. The wiggle room is greater at longer wavelengths.

Loop shape and positioning above the reflector disc both influence the match and radiation pattern, as indicated in the accompanying article you posted. My brief posting for the combined 2.4/3.4 GHz loop feed provides figures for construction. I tried to make the loops as circular as practical, and kept them concentric. Even with just two bands, it required a lot of patience to tune, such was the inter-action.

It's been a while, but I recall basing my construction on an article I found somewhere on the web. I might still have it. It made no mention of a collar around the reflector, but your posting has me wondering if it might provide a further improvement in performance.

As Adam pointed out, any kind of multi-band arrangement will be a compromise. Indeed, even a mono-band feed is a compromise. The trade-off here is performance vs. convenience, of course.

Regarding microwave activity, it seems to have migrated south across Bass Strait, with Hayden VK7HH and company leading the way.

Chas
VK3PY

[VK3QI]

As a long time successful user of RF HAMDESIGN 1/2 Dual Band, 3/5/10 Triband and 1/2/3/5/10 5 band feeds I would like to make a few points.

(1) Moisture issues can be alleviated by simply drilling a drain hole in the bottom of the feed. I note the LA boys were using a Gore-Tex plug, no doubt to stop insect ingress when the feed is permanently mounted outside.

(2) The damage shown to the 1.2 feed was no doubt caused by a poor quality N plug and overtightening, resulting in the center pin of the N socket turning.

(3) The size of the reflector (17 cm.) reduces the radiation efficiency due to blocking when used as a center feed with a dish 80cm or less in diameter.

(4) Used with dishes of 150 cm. to 240 cm. diameter, center feed works well on all feeds, however experience suggests that 10 GHz. beam width becomes far too narrow for reliable portable work, unless you can adjust both vertical and horizontal positioning.

(5) I have found the return loss on each feed to be well within specifications in all cases, regardless of the positioning of the feed.

(6) Used with offset feed dishes, the limitations of blocking on small dishes is eliminated and performance is excellent.

(7) Isolation between loops is, at best 16dB, which means that care must be taken in protecting the receive input on bands not being used. There are several ways of doing this:

(a) ground or terminate in 50 ohms receive input relays not being used
(b) connect all unused loops to the transmit side of each transverter.

My preferred method is the latter as it reduces the number of relays involved.
To quote from the RF HAMDESIGN literature: “When you use this feed as a multi-band feed at the same time, RX/TX, it’s recommended to load the unused ports to 50 ohms or use a bandpass filter – cross talk is 16dB.

(8) The recommended feed position (as recommended by RF HAMDESIGN) is 2.5 cm. behind from the front surface of the feed and corresponding to the phase center for the highest frequency used.

(9) The specifications for the V and H -3dB and -10dB range vary by a factor of approximately 1.5
For example 1.2 GHz V -3dB = 72 degrees 10GHz V -3dB = 44 degrees

(10) The feeds are optimized for a prime feed of 0.45 to 0.5 F/d dish.

Most prime feed dishes available in Australia will have an F/d of around 0.4
Most offset feed dishes available in Australia will have an F/d of around 0.5 – 0.7.

This means that results can vary significantly, depending on the particular dish being used.

(11) Power capability of the loops is dependent on the type of connectors being used. I have had no issues with 200 watts on 1.2GHz, 100 watts on 2.4GHz, 50watts on 5GHz and 30 watts on 10GHz.
I hope the above points help to understand the benefits of the RF HAMDESIGN feeds.

Cheers

Peter VK3QI and VK3ER/p

[VK4AFL]

I purchased one of their dual band [23 cm & 13 cm] feeds which to the door cost $262 Australian. The spec sheet claimed 30 db return loss on each band which to me immediately sounded suspicious. The reality was 16 db on 23 cm & 12 db on 13 cm in free space. It was then measured again in front of a reflector with virtually the same results. Not being too hung up on SWR the next test was installing the feed on a 2.3 m dish with a suitable FD ratio then comparing it with a yagi array on 23 cm & a grid pack on 13 cm. To cut a long story short performance of this feed on each band was very poor. In the case of 13 cm the on-paper gain improvement should have been approx 9 db but instead was about 3 db. I didn`t bother to consult RF HamDesign as it would not have led to anything positive. My particular example which did not illuminate the reflector properly was expensive junk. Would anyone like to buy it?

73, Trevor, VK4AFL.