3D Corner Reflector for Pulsars

[Coto]

Hi all,

 

We're currently working on the 3D Corner Reflector antenna for pulsars. I emailed the creator of the original antenna but I haven't gotten a response in a while (maybe he's been busy).

Here are my questions if anyone is able to help:

1) http://iw5bhy.altervista.org/info.php here the creator of the antenna says the monopole is 53 cm long. Why so long? Doesn't theory say that the monopole's length should be around λ/4 = 70/4 = approx. 17.5 cm?

2) Why so thick? 10mm sounds pretty thick to me... On feed designs for the hydrogen line I've seen thicknesses less than 3mm. How does this huge thickness help?

3) Also, how would I manage to solder this huge thick wire onto an N-Type connector? The N-Type connector's pin is far smaller than 10mm...

4) Does the ground side of the N-Type connector have to make electrical contact with the metal grid? Is a simple solder to one of the wires enough, or must I make sure the entire connector makes GREAT contact with the grid (i.e. put a metallic plate for the connector to sit on)?

5) Do the metal panels (each 2x2 side) have be in electric contact with each other (e.g. a wire or two touching between each panel)?

6) Does the antenna's ground need to be in contact with the Earth (at this frequency will good Earth grounding provide any reduction in static/noise)?

 

Thanks a lot!

[robin_astro]

any multiple of 1/4 wavelength will resonate

Edited July 24, 2019 by robin_astro
deleted

[robin_astro]

The ratio of antenna length to thickness affects bandwidth. A thicker antenna element increases bandwidth 

[robin_astro]

What is inside the box the antenna element is mounted on ?

What is inside the box ?

[robin_astro]

The chicken wire should be connected along all edges and to the ground plane plate and outer of the coax connector. See page 20 of this paper for example for tips on building a 3D corner antenna

https://www.qsl.net/yu1aw/ANT_VHF/shorten3dcra.pdf

 

[StarryEyed]

1. There is a relationship between the length of an ariel and its resistance (impedance) a half wave length gives 50 ohms so the coax that connects the ariel to the radio must also be 50ohms. TV coax is 75ohms...... 

3. With wire inbetween

4. Yes it can be bolted to a plate and the ntype conected through the plate.

5. Ideally yes.

6. No its not that kind of earth given the sides are acting as reflectors. This is more like a yagi or satellite dish.

You sometimes see TV ariels with a V shaped reflector at the back rather than the flat plate like reflector. Its a similar thing. It looks like a pain in the rear end to make and direct.

Robin astro.

Probably nothing if the ariel has the same impedance as the coax and radio. Otherwise a couple of components to change the impedance. To make it match.

Edited July 28, 2019 by StarryEyed

[robin_astro]

1 hour ago, StarryEyed said:

Robin astro.

Probably nothing if the ariel has the same impedance as the coax and radio. Otherwise a couple of components to change the impedance. To make it match.

Yes  I suspect  some kind of matching arrangement otherwise why is it there rather than mounting it directly to the ground plane ?  Perhaps the element just slides within the box to tune the length to resonance   (The 53cm long element  is  an electrical 3/4 wavelength long  to resonate at 70cm when mounted on the ground plane.)  I would want to know before building this though

Cheers

Robin

Edited July 28, 2019 by robin_astro

[robin_astro]

1 hour ago, StarryEyed said:

a half wave length gives 50 ohms so the coax that connects the ariel to the radio must also be 50ohms.

Actually a half wave dipole in free space has an impedance at the feed point of 73 ohms.  though that depends on the height above ground. Adding parasitic elements also reduces the impedance.  In the article I referenced above, the 72 ohm impedance of the 3D corner antenna similar to this one with a 3/4 wavelength element was reduced to 50 ohm by adding an additional parasitic element, though this design does not have this.

Robin

Edited July 28, 2019 by robin_astro

[Coto]

My suspicion from what I read from the link I gave is that in the box there might be the pre-amplifier. I'm not 100% sure but that's my guess since there's nothing else to be there according to the block diagram given below in the page. I did a few simulations in CST Studio and found that a slightly shorter and thinner wire does the job for me (just by looking at the S-Parameter plot). Is there anything else I should look at to confirm this 50-Ohm impedance match?

[robin_astro]

13 hours ago, Coto said:

My suspicion from what I read from the link I gave is that in the box there might be the pre-amplifier. 

 There are two amplifiers shown in the block diagram but the one connected to the back of the ground plane appears to be the first one "LNA 03-70cm from US4ICI (NF = 0.3dB, commercial solution)" visible in the image here rather than the second "LNA FOR ALL"  shown in the block diagram at the other end of the feeder before the SDR dongle

 

Of course matching to 50/70 ohms is not super critical for receiving but it would need to be tuned to resonance for efficient operation and with that box there it is uncertain where the length of the element is referenced from

Cheers

Robin

[StarryEyed]

15 hours ago, robin_astro said:

Actually a half wave dipole in free space has an impedance at the feed point of 73 ohms.  though that depends on the height above ground. Adding parasitic elements also reduces the impedance.  In the article I referenced above, the 72 ohm impedance of the 3D corner antenna similar to this one with a 3/4 wavelength element was reduced to 50 ohm by adding an additional parasitic element, though this design does not have this.

Robin

Yes thats all true but the ariel isnt in free space its on the earth and so it ends up close to 50. Keep it simple.

Edited July 29, 2019 by StarryEyed

[StarryEyed]

The diagram for gain shows about 11db for one wave length. For hydrogen its recive only I would rather make a 1.2 ghz yagi make an array and a RSP2. 

Edited July 29, 2019 by StarryEyed

[robin_astro]

2 hours ago, StarryEyed said:

Yes thats all true but the ariel isnt in free space its on the earth and so it ends up close to 50. Keep it simple.

This isn't a half wave dipole either 😉

[robin_astro]

1 hour ago, StarryEyed said:

For hydrogen its receive only I would rather make a 1.2 ghz yagi 

This is for pulsars at ~70cm though.  For Hydrogen there is an interesting practical comparison of a yagi (actually a helical) v a small dish early in the Hydrogen line thread. The dish won out in this case.

Cheers

Robin

(G8DVW)

 

[StarryEyed]

The article here.

https://www.neutronstar.joataman.net/amateur_challenges/

Talks about gain later on and suggests that arrays in the ghz offer more gain for a smaller antenna than bigger dishes in the mhz. This  based on smaller yagis with higher gain in arrays all more easily mounted. Plus it doubles up for eme were its tuning and operating can be refined and practiced. Maybe not intuitive but listed on the site are amateurs getting results this way.

Personally I find this idea more compelling, practical and its potential given its amature radio origin.

But lets be honest none of these things are easy or cheap. As I am currently painting the shack (back room to the garage) with everything in boxes it has dawned on me I have more ideas than time to be able to fullfill them!!

Holidays for a start.

Kevin. M0LKJ