Adapt 13cm gridpack for 23cm? Advice please

[VK2GOM]

I found an old gridpack dish in my garage, with a 2.4GHz (I think) feed on it that never worked too well on 13cm ATV, hence the plastic cover broken off in an attempt to see what might be wrong with it.

I'm looking to modify this antenna for 23cm FM use. Any advice on how to proceed? It's fed with decent coax and an N connector. There are no signs of a balun; it looks like the current brass strip dipole is terminated right off the end of the coax

Any advice on feeds that might work, and whether I'd need a feed reflector etc. welcome.

73 - Rob VK2GOM / GW0MOH

[VK4GHZ]

Hi Rob

That certainly looks like a bog standard consumer 2.4 GHz WiFi cast aluminium grid pack.
Can't help with 23 cm scaling (should be straightfoward?), but...

with a 2.4GHz (I think) feed on it that never worked too well on 13cm ATV, hence the plastic cover broken off in an attempt to see what might be wrong with it.

Was there ever a reflector element?
ie; it's really a 2 element yagi pointed at the reflector.

A lack of a reflector would result in a poor F/B and therefore poor illumination of the reflector, and therefore poor forward gain (the bore sight of the actual gridpack).
These reflector elements are rather fragile, held in only with a single self-tapper.

Some existing non-specific images I already have on-line, where what I'm referring to is clearly visible.

[VK2GOM]

Hi Adam, yes, it did have the bent Aluminium reflector originally.

I might measure the elements that are there now, and see how the 1/2wave calculated out to a frequency. I could even leave this existing dipole on, and solder more brass strip to it to extend it for 23cm.

I would then need some sort of box around it all, and make up some kind of slightly larger reflector, or even a calculated (Yagi software) reflector element for length and spacing.

I'll report back in due course with results of the experiment, but it would be good to hear if anyone has built their own feed for a gridpack on 23cm, that works!

73 - Rob VK2GOM / GW0MOH

[VK4UH]

G'day Rob

You might consider the use of a "Bowtie" feed for 23 as was used on the VK9NA DXPedition to Norfolk Isalnd.

The attached image has one on a 1200 mm dish but the same principle would Im sure be applicable to a moderate sized GridPack.

The gain would not be particularly good on a gridpack this size at 23 cms and I wonder if you would be better of with a Yagi at this frequency?

Kevin VK4UH

[VK2GOM]

Hi Kevin, yes, it's only 80cm or so this dish. I see the gridpacks Radio Specialists used to supply were 1m.

Some further probing around looking at the existing feed, it turns out it does employ a gamma type match further inside the tube. Of course, the gamma length would be particular to 2.4GHz, so it's not simply a case of extending the elements. I'd have to start from scratch if I went this direction.

I might have a Google and see what I can find about double diamond / bowtie type feeds.

73 - Rob VK2GOM / GW0MOH

[VK3MIX]

I have one of these 2.4GHz Gridpacks which used to be used for Wifi back in the day.

Do they need any modification to work on 13cm?

[VK2GOM]

I used it on 13cm, although it did have some issue with the feed. Aiming to a line of sight station, I had to point the dish about 40deg elevation, and point off to the right by about 30deg.

I got a replacement dish, and that one works on-axis.

73 - Rob VK2GOM / GW0MOH

[VK4KAY]

There are grids and then there are grids. I have tested numerous types 10 years ago for commerical purposes in 2.4 and 5.8 data links with low power.
They had to work 24x7, survive wind, rain and salt air of NQ.
Let me generalise here for simplicity.
Grids that come with feeds using coax tails of RG58 or RG213 - dont bother.
Grids that use semi-rigid coax or have connectors directly connected to the back of feed - take a second look.

The grid pack photo above I think Adam posted is a good quality type. Powder-coated Aluminium, rigid - maintains its shape, lets the air through, lasts in the corrosive air etc.
These work quite well at 23cm and 13cm.

I use to purchase ones that were made in the USA - I think by a subsiduary of Cushcraft (just cant remember) - the feed used a printed circuit matching system close to the dipole at
the front and it had a SOLID sub-reflector - no gaps like the photo above. The feed would handle at least 50 watts.

I have used one of these grids on 23cm for more than 10 years. I modified the feed by soldering extra wire to the ends of the dipole and matching it accordingly in a stub like manner (ie. some of the wire hovers over the printed board.) To increase the BW I used 36mm2 hard strand electrical cable and drowned it in solder to seal it. Silicon or glue or whatever can be used to hold the "stubs" in place. Then the plastic cover (radome) is used to cover it and I used elec tape and silicon to do the rest. Water and dirt (dirt is any foreign material that can affect the feed - including bird "business") is an enemy.

Used for terrestrial use only - this antenna has provided contacts on 23cm from Cairns to Brisbane when I was in Mackay using 10w and 40w.

I dont have a 13cm setup yet - but when we used these antennas on 13cm for data links - if there was a troubled path due to objects or high RF levels - this was the go to antenna.
I will upload my vswr curve for 23cm in a later post.

[VK4KAY]

Grid Feed for 23cm Wide Freq VSWR

[ZL1TPH]

Help - I am about to purchase one and just mesh out the grids (with fine mesh that is) to make work up to 3.4 GHz. What is the F/D ratio of these so I can look at a suitable feed.
Cheers, Steve

[VK3QI]

Steve,

FWIW I have a 2.4GHz grid pack as a FD backup in storage which has a long side width of 980mm and a focal length of 330mm.

This gives an F/d of 0.34

I am not sure about the smaller grid packs, but from sight they look to be similar.

Cheers

Peter VK3QI

[VK4KAY]

Steve

I have measured a couple of grids I have here. I will post some photos shortly with measurements.

However depending on the type / brand I am seeing a variation in the F/D.

I think the mainstay use 0.34.

However the one I use is 0.39. I also have others that are 0.36.

Andru
vk4kay

[VK4KAY]

1st photo shows the difference between grids designed for 2.4GHz and 5.8GHz - just for anyone interested.
I think the difference is obvious. Tighter spacing on the slats.

2nd photo shows 2 different 2.4GHz grids. Here are some measurements but NOT highly accurate. Cast grids are good for repeatability of design.
The grids on the right vary in dimensions by millimetres due to shipping (they get out of shape) etc. At lower freqs this shouldnt matter a great deal.
I think focus on connector and cable losses and quality of those before worrying about 1 or 2mm at 1.3Ghz with reflector antennas.
Grid on the left is 992 x 600mm - running the tape down the surface - 1075 x 615mm - feed position to centre of dipole above surface is 390mm
Grid on the right is 842 x 594mm - running the tape down the surface - 927 x 615mm - feed position to centre of dipole above surface is 290mm

The 5.8GHz grid just to post the information is 865 x 595mm - running the tape down the surface - 950 x 611mm

The 3rd photo of the cute little 5.8GHz antenna dimensions are 382 x 245mm - running the tape down the surface 411 x 251mm