RH Ham Design Ringfeed - a quick look

[VK4GHZ]

Here is a quick review at the RF Ham Design tri-band ringfeed for 9, 6 and 3cm, model # FPF R963, and accessory mounting bracket, CLX-1.

Although RF Ham Design produce quite a number of parabolic dish ringfeeds, and already having a gridpack for 2.4GHz, the tri-band feed for 3400MHz, 5760MHz and 10368MHz was of interest.
The total shipped cost, (which included the optional CLX-1 mounting bracket), via registered mail to my door was 201.50 Euros.
With the relatively strong Aussie dollar at the moment, it's a great time to be buying from the USA and EU.

Why is a muti-band dish feed of interest for Field Day use?
KISS!
Four gridpacks and/or dishes for 2.4, 3.4, 5.7 and 10GHz would simply not fit into the vehicle, not with all the other radio and camping stuff anyway!
And, who wants to align four different antennas each time to work the same station?


Above: 13 days to get here from the Netherlands, not bad!



Above: Well packed - double-boxed!






Above: Separate SMA connectors for each band






Above: For horizontal or vertical polarisation, align accordingly



Above: A network analyzer plot is provided for each band



Above: View from the "business end"

There is no mention in the documentation of where the focus point is.
I sent an email to Frank asking where the focus point was, in relation to the front face.

Hi Adam,

The center of all bands is in the middle of the feed, for calculating focus point, add 3.5cm
to get the reight focus distance compared to front of the feed.

Kind Regards,

Frank Postmus
Chief Executive Officer
Room H2/B

RF HAMDESIGN
The Netherlands

So, 3.5cm in from the front face, towards the rear face with the SMA connectors on it.



Above: Front end is tapered. The front face (not sure of material, but looks like fibregalss) appears to be glued on, for a weatherproof seal



Above: RF Ham Design CLX-1 ringfeed mounting bracket



Above: Stainless hardware


Above: This visualizes how the tapped POM (polyoxymethylene) pieces attach to the aluminium strap



Above: For a prime-feed dish, this is how the three feed arms would attach to the CLX-1 bracket

To mount to my 65cm offset dish, I will be fabricating a custom mounting arrangement to fasten the CLX-1 bracket to the LNB mounting arm.


Compared to optimized mono-band feed and dish setups, a compromise is expected, but the convenience and practicalities for Field Day use far outweighs any negatives.
It might be a different story with EME, trying to squeeze the last 0.1dB out, but for FD use, let's keep it real, and just make contacts.
In this case, you align the dish once at 3.4GHz, and the antenna is already aligned for the contacts on the higher bands.
(Which would become increasingly difficult with the narrower beam widths involved at the higher bands)

Ordering with RF Ham Design is easy, one email to confirm total price (essentially to confirm the shipping costs), and they accept credit card and PayPal.
There is a small % surcharge for credit card and PayPal, but don't forget to take off the 17% VAT (applicable to internal Netherlands customers only) from the prices on the price list.
Frank responds to email very quickly, even allowing for the time difference between Australia.

It will be fun to experiment with and use this.

[VK3APW]

Hi Adam,
very interested to know how the triband feed on the offset dish is performing. It makes great sense field day ops. As you say, you just cant get all that gear in your car. Do you have any way to compare it against a 'mono' band feed?

Regards
Peter vk3apw

[VK4GHZ]

It's been a few months , but to finish the topic, I have now mounted the RF Ham Design ring feed to a 600mm prime focus dish, that has an f/D of 0.4.



This surplus dish was provided by Alan, VK4NA, from the Queensland Digital Group.
A big thank you to Alan... and as you can see, has gone to a new home that has done something with it!

The CLX-1 mounting bracket makes it very easy to mount to the rim of a dish.

Using W1GHZ's HDL_ANT.exe package, we determine that the focus point for this particular dish is 242mm.
Note that HDL_ANT is an old package, and will not run on Windows-7 64.
It does run ok on an old Windows-XP box.
Hopefully Paul will be able to provide an update to this excellent package, so it can be run on newer operating systems.

Not having done this before, I suspected the key to success was: symmetry.
Keeping everything symmetrical within the bounds of a sphere, and it all should line up.



Above: Using household items so the feed can sit at the correct height whilst measuring up



Above: Three identical mounting arms were fabricated out of 12mm "U" section aluminium from Bunnings (less than $5)



Above: The POM (polyoxymethylene) cutout suits 15mm aluminum available in Europe.
16mm section available here is too big, so 12mm section is used, along with shakeproof washers to take up the gap.
The POM brackets will "clamp in" for a nice tight (and rigid) fit.



Above: now we can mark holes at the rim end of the brackets



Above: Mounting detail at the rim end.

L brackets (20mm wide) were cut from 20x12x3 aluminium and tapped with M3 threads.



Above: Brackets were cut so that the ring feed would sit in the middle of the range, so that a focus point distance tweak could be made.





A test using a portable microwave signal generator was performed.
With the generator 500m away, sitting on a car dashboard pointing back to base, line of sight to a near by hill, a test was done.
(It is important to get out of the near field, when lining up dish feeds.)
FSD!
The directivity of the dish was excellent, reasonably sharp, and agreed with what I could sight by eye.
If an antenna has directivity, it has forward gain.

The signal generator was actually operating on 1700MHz, and we were listening to the second harmonic.
At 3400MHz, the output is +7dBm (5mW), as measured on a HP8562A spectrum analyzer.
The gain of the WA5VJB log periodic is not known at 3.4GHz, but I would not expect much.

The log periodic antenna was removed from the signal generator, and 50 ohm SMA termination then became the antenna.
This then produced a weaker signal to recheck alignment of feed attitude and focus distance.
(Consider the feed is in 3D space)
A small peak was detected by ear at the designed point.

I have not yet tried this tri-band feed dish arrangement on 5.7 or 10G yet, but will do so as the extra transverter systems are built up.

Following the procedures in the W1GHZ Microwave Antenna Book has provided a system that appears to work as expected, with minimal fuss and black magic.

[VK3HZ]

I went through a similar exercise recently with a 380 mm Mitec dish for 24 GHz using a Procom prime-focus feed.

The program gave 147 mm as the focal length. However, sun noise peaked at 156 mm.

So, as you have done, it's wise to make the feed adjustable so that you can peak it on a real signal.

Regards,
Dave.

[VK5ZD]

Hi

How much coupling is there between the antennas?
I wouldn't like to transmit on, say, 10GHz and discover I've killed the front end device in the 3.4GHz and/or 5.7GHz transverters.

73
Iain

[VK3ALB]

Adam,

Did you use the feed on the weekend? How did it perform? Like Matt I'd like to know how you connected to the feed. Considering you have three transverters in one box I assume you ran cables to the feed.

[VK4GHZ]

Quickly catching up on a few questions.

How much coupling is there between the antennas?

Iain, from the manual;

When you use this feed as a multi band feed at the same time RX/TX, it's recommended to load the un-used ports to 50 ohm, or use a band pass filter !! (cross talk is about -16dB)

Where are you going to mount your transverters? One on each arm of the dish feed? Or are you running cables up to the feed?

Matt, I have a dual-band 2.4/3.4 transverter system in one 3RU 19" rack box... pictures found on this forum.
I run short lengths of RG214, separate feedlines, for both 2.4 (to a gridpack) and 3.4 to this tri-band arrangement.
Both have N connectors at the transverter end.
The RF Ham Design feed has SMA (f) connectors, and I use an SMA (m) to N (f) adapter.
Some purist might turn their noses up at this, but at the end of the day, the 1.2 dB loss on 3.4G doesn't bother me!
For Field Day use, practicality over everything else, and it isn't EME where every last 0.1dB matters.

Did you use the feed on the weekend? How did it perform? Like Matt I'd like to know how you connected to the feed. Considering you have three transverters in one box I assume you ran cables to the feed.

Lou, never got used on 3.4, which is the only band I am using it for at the moment.
Conditions too bad.
Very disappointed.

In fact we never made any contacts on 2.4 either... in exactly the same spot we made many armchair copy contacts only two months earlier during a local microwave activity day.

For 5.7G and 10G, I will have to rethink this, and probably move the feed onto an offset dish, so the 5.7 & 10G transverters enclosures can be mounted on the LNB arm, and close to the feed SMAs.
Feed the 5.7 and 10G with short lengths of 141 semi rigid then.

I could always get another CLX-1 mounting bracket, and move the feed from one dish to another quite easily, and quickly.

[VK3XDK]

Another old post
I have been playing with a very similar feed,
I suppose it would be called a DL4MEA feed http://www.w1ghz.org/antbook/chap6-2.pdf.
In "free space" i am getting VERY good return losses on my version (-30 or better) but as soon as i put it in front of a dish the best return loss i can get when the feed is in the correct focal position is around -10db (the phase center is mentioned to be just behind the ring in the for mentioned link)

I have noticed that if i bring the feed in- maybe 30mm or so (towards the dish) i can once again get great return loss results, i wonder if there is a bit of "cheating" with RF Ham design's phase center? Or maybe i'm just doing something wrong?
I would guess that their supplied test sheets are "free space"

I wonder what return losses people are getting with these feeds once positioned- how are they adjusted for minimum return loss if things aren't right? , has anyone played around with the focal position?

30mm focal difference at 1296 (for me) probably what make much difference in antenna efficiency but i would like some "experienced" comments

[VK3QI]

Graham,

I have a number of the RFHAMDESIGN ring feed systems.

(1) 1.2/2.4 feeding an offset 1.2 metre dish

(2) 1.2/2.4 feeding a 2.4mx prime focus dish

(3) 3/5/10 triband feeding a 900mm offset dish

(4) 1/2/3/5/10 5 band feed - yet to be deployed.


I have not noticed any significant deterioration in the return loss on any of the systems when deployed on the dishes at the required focal points.

(A) what is your method of measuring - could there be some sort of detuning by your cable connections and positions?

(B) what size reflector plate are you using? The RFHAMDESIGN plate is typically about 16 cms diameter.


Cheers

Peter VK3QI

[ZL2BKC]

Another old post

I have noticed that if i bring the feed in- maybe 30mm or so (towards the dish) i can once again get great return loss results, i wonder if there is a bit of "cheating" with RF Ham design's phase center? Or maybe i'm just doing something wrong?
I would guess that their supplied test sheets are "free space"

Graham,

My suspicion for the poor SWR is RF bouncing back off the centre of the dish and entering the feed. Think of a lossy waveguide with a short at the end.
What can you do?? - firstly confirm this is the problem using a sheet of aluminum and check it repeats every half wavelength. I have seen this with my 3GHz dish feed during testing on on the bench, but since I use an offset dish for that band the feed never sees itself.

What I have seen is a small conical disk similar in size to the feed horn placed in the centre of the dish to ensure the RF does not reflect back 180 degrees into the feed. If done correctly it will not change performance as the feed naturally interferes. Maybe a small piece of absorbing material (black anti static material) will help reduce the effect.

73,
Wayne ZL2BKC

[VK3XDK]

Thanks for the replies, i've had quite a few (including email)
I ended up changing dishes and never "got to the bottom" of the original dish. (crappy? old "whale bone" wifi dish)
i am now on a smaller? (0.84) offset fed dish, although fd is bad- i have seen much better results. Local beacons way up and return loss seems to be much less affected.

As one reply pointed out- it must be a reflected signal issue with my return losses. Good or bad?-- puts a lot of doubt in getting things right!
Anyway,

I had a wsjt play with a BIG EME station but saw nothing, could be many reasons for this and i am now thinking i might defer 1296. ( 10ghz looks a much more realistic option with my dish-space restrictions)


Be great to have a "blue dye" to see what is really happening one signals reach the antenna!