Another tool for planning LOS contacts

Following a previous post on tool for planning microwave (outdoor) activity, I would like to add a web service that eases the search of two spots in line-of-sight: https://link.ui.com .

Select a starting point in the map (single click), then "+ Add PtP" (Point-to-Point). Then drag the grey circle (receiver) around and pay attention to the color of the line: green means OK, red means NO-GO.

From the bottom of the page a slide-up appears which shows the terrain profile below the RF link and antenna heights. Don't forget to review antenna heights because the default value is 12 meters above ground!

Remember that the tool does not take into account buildings and vegetation!

Not all CDM324 are created equal

Since there is probably no way to use my three specimens of CDM-324 24 GHz radars, I decided to open one of them. Remember there is a well done teardown of a similar module on youtube, which I use as a reference.

The lid is glued and comes off sliding a blade all around the perimeter. Don't try to pull or lift, it doesn't work.

Surprise: there is no RF sponge in the lid vs. the module shown in the video. Also the PCB layout is slightly different, some vias are not present either.

Unfortunately there seems to be no identifiable physical sign that helps spotting different modules.

AS-IS CDM324 is unsuitable for 24 GHz QSO

I tried receiving the 24 GHz signal at 4 metres distance pure line-of-sight inside a room. The receiving end IF port was sent to an RTL-SDR receiver, while the transmitting end was modulated with a WBFM injected into the IF port or CDM-324 (a.k.a IPM-165).

The signal was quite weak, about +10 dB above noise and quite spread. Considering that I received an HB-100 with an HB-100 at 300 metres without the aid of extra antennas, I can state that the CDM-324 AS-IS is not suitable for WBFM (ham radio) operation on 24 GHz. This is actually the information I was looking for and did not show up in my searches.

Maybe there is a loose LPF under the hood, as shown in a video on youtube. It could explain with the signal with 50 MHz difference is weaker than the one at 25 MHz delta.

I will open one of my CDM324 and try to make it as similar to an HB-100 as possible.

Modulation through IF port on CDM324

Since the frequency difference of a couple of my modules falls within my SA/TV-test-set receive range, I could try WBFM modulation. I used the same modulator of my HB100 10 GHz transmitter, without decoupling-smoothing capacitor across +5V/GND. The audio was signal fed through the IF port.

Modulation did work, but I was probably overdriving something because the spectrum looked awfully wide and jagged.

CDM324 can be modulated on the +5V line too, so there are two ways to do it.

But .... it is more than 3 times as sensitive as an HB-100, so much more care must be taken on the electronics side.

First check with CDM324 couples

As I did on 10 GHz back in Spring 2017, I checked the frequency difference between my 3x CDM324 modules. Currently I have no means to measure a frequency at 24 GHz, so I trust that they are in the ballpark.

Test setup is as follows:

• put two radars in front of each other at 20-40 cm distance
• power them up and pick the IF output of one CDM324
• feed the IF output signal to an
• oscilloscope
• frequency counter
• spectrum analyzer

Samples 1-2 were visible on the 'scope and the counter, about 25-30 MHz apart.

Samples 1-3 were not detected on the first two instruments, but the SA put them about 70 MHz apart.

With the couple 2-3 I should be able to understand how they are spread around 24.125 GHz :-)

This result also confirms that there is no low pass filter on the IF output of these things, so they can be used with an external "receiver" to demod WBFM.

Size of CDM324 vs HB100

Three CDM324 24 GHz radars have arrived. This is the size comparison vs the 10 GHz counterpart HB100:

The CDM324 has no holes for attaching it somewhere, so you need a daugtherboard behind it which acts as a driver too.