Seductive serendipity / Verleidende serendipiteit

August 8th, 2017

PraatPaalPatch (PPP) for 23cm FM-repeaters

Note: For the impatient click here and here.


For some years The Netherlands owns nationwide covering 2m and 70cm
FM repeater systems (2m PI3UTR and 70cm PI2NOS).

Note: some links are in Dutch only, sorry.

In Dutch HAM slang a repeater is called ‘PraatPaal’ (Talk Pole <– direct translation ; -)

Availability of cheap Chinese handhelds like ‘Baofengs’ stimulate pollution on 2m/70cm.
The repeater systems attract lots of morons and/or trolls generating interference.

Combined with the fact that such a repeater system has a capacity of less than one (1)
Erlang makes ‘our’ nation wide repeater systems a sociological and/or social experiment.

An observed trend is that Full licensed HAMs tend to avoid ‘our’ nation wide repeater
systems because they don’t want to be associated with trolls and try to find their salvation
in other areas and/or aspects of the hobby.

One ‘escape route’ for repeater usage is to increase frequency, for which 23cm is a willing candidate
in my region. Novices are not allowed to use this band here and little (commercial) equipment is available.
This filters out unwanteds and trolls significantly.

A few years ago Bas PE1JPD designed his 23cm transceiver to stimulate activity on 23cm.
Bas’ project is quite a success and he was seduced to publish his project in FunkAmateur (iirc July 2017 issue).

Now (relatively) lots of Full Licensed HAMs here build 23cm NBFM transceivers and need a 23cm antenna.
In my region the 23cm repeater (PI6NOS) is located on a 160m tower and within LOS (line of sight) for many.

Simple 23cm antennas
Lots of designs of 23cm antennas are available and possible. A very popular/easy/simple antenna
is the Doppelquad (German, advocated by DJ9HO (sk) in the 80/90′s) or BiQuad (English).
And, yes/indeed, the Doppelquad is easy to construct (Google on it or see below).

Fig 1. Doppelquad (German) or Biquad (English)

However . . .

Although the Doppel/BiQuad is easy to construct, there is (now!) a competitive alternative:

The Patch Antenna -aah -aah! (or patch in short)

Patches are far beyond new. In fact, patches are old skool. They are/have been widely used
in all kinds of wireless systems like RC, WiFi, GSM, GPS, DECT etc, but queerly ‘underexposed’
in HAM scenes, despite perhaps (am)satellite usage like AO-40 (sk).

Although theoretically ‘free space’ patches have around 2 dB less gain compared to a
Doppelquad (Biquad), building a patch antenna is UTTERLY simple!

(perhaps too simple for the ‘common HAM scene’ ?)

In The Netherlands (most) 23cm repeaters have 28 MHz shifts: e.g. output, input MHz.
This means the patch has to be designed for 1270 MHz (50 + j0Ω), i.e. at the CPE transmit frequency.

I found one 23cm patch effort from G6GVI (1296 MHz) via Google.
However, this is a FR4 ‘dielectric design‘ (with according dielectric losses).

So, I calculated and built a ‘full size’ patch from scrap material for 1270 MHz, i.e. a patch with εr = 1 = air.

The result is screwed on a bookshelf in my shack, and indoors pointing at PI6NOS
(1270.375 in / 1298.375 MHz out). Below a picture of the contrapsion (click to enlarge in new tab).

Minimalistic PI6NOS 23cm repeater antenna setup, enough for regional work ; -)

In my previous post I built an Inmarsat patch according to 9A4QV’s dimensions.
I mailed Adam my interpretation. His response was:

“The patch should not be made of double or single layer FR-4 copper clad.
It should be made of a conductor plate only. You are introducing extra Er in the
air gap together with the extra copper layer. This will not work properly.”

Of course I considered this while building the antenna, but I do not agree
with the above statement because both sides of the PCB are electrically connected
on two places: the centre (Z = 0) and the feedpoint.

Anyway, this (thin, 0.6mm or so) FR4 PCB material has been crying for some usage
here for almost 30 years ; -) Of course brass or copper plate material suits perfectly.

Unfortunately I was not able to measure my Inmarsat patch because a good friend
almost immediately ‘confiscated’ my built. He currently uses my patch (or Adams ; -)
on a camping site somewhere in the neighbourhood to receive Outernet.

While typing this I received a phone call from him, mentioning he got his Outernet
setup working with his CHIP and has an SNR of around 6 dB with my patch.

Of course this is not an objective figure of merit. However, I was able to measure
my PraatPaalPatch this week and consider the results representative for my
interpretation of Adam’s Inmarsat patch because the two patches:

- operate in the same frequency region (1.27 vs 1.54 GHz), therefore . .
- have almost indentical dimensions
- are made from the same materials

The difference between the two patches is the polarization. Despite perhaps axial
ratio issues (Inmarsat patch), construction of the two patches is almost identical.


The PraatPaalPatch was measured at two locations using two different setups:

- PI4RCG radioclub, R&S spectrum analyzer
with tracking generator and -20 dB directional coupler

- work, Agilent Network Analyzer

First location
When first measuring the PPP at our radioclub results were not promising.
I could hardly see a dip (around 1270 MHz). With some fiddling I discovered that the
dimensions of the patch plate were okay, so it had to be something with the match.

The nature of my fiddling led me to the conclusion that the patch-reflector distance
was too small. Initialy I calculated 12.5 mm (say 1/2″). By increasing this distance
with a M4-nut and pull out the feedpoint wire (so at this position the distance was
also increased with around 3mm) the patch came to life !

Below the result from the radioclub measuring session is depicted (click to enlarge).

Measuring the PraatPaalPatch at PI4RCG Radioclub (Photo by PE1PIP)

On the picture above it can be seen that the match @1270 MHz is almost perfect!
Return loss is around 42 dB (!) when pointing the antenna towards the ceiling.

Second location
At work the setup below was created (see picture below, click to enlarge)

Fiddling a little with the antenna revealed an even higher return loss of around 50 dB (!)
(see below, click to enlarge) but the ‘average’ RL was around 36 – 40 dB, which is very good.

PraatPaalPatch dimensions !

For Fres = 1270 MHz construction details are visualized below (click to enlarge)

Front view                                                          Side view                                                    Back view : -)

In text: reflector plate 180 x 180mm 2mm think Al-sheet (not critical), patch size 103 x 111mm
brass/copper plate (around 0.6mm thick, I used double sided FR4 PCB material). Patch centre
(determine with diagonals) is grounded with a 16mm M3 metallic spacer (I used a 1/2″ spacer + M4 nut ; -)
and feed point is in the middle of the short (103mm) side and 9mm from this patch edge.

Connector used here is N-female with flange. Wire from center pin of connector is around 2mm
diameter and fits neatly IN the center pin of the connector : -) In case double sided FR4 material is used,
solder feed point wire on both sides to ensure electrical connection. Feed point –  reflector distance is also 16mm.

Note: when the longer patch side (111 mm) is horizontal, antenna polarization is vertical.

PS. When you refer and/or build this patch and publish about it, please refer to this article and/or my callsign.
I do the same when I build stuff from others!

August 6th, 2017

L-band/InmarSat/Outernet patch antenna cf 9A4QV

In my previous post I elaborated somewhat on Outernet being merely
a ‘nerdy’ technological project. Adam 9A4QV designed a L-band patch antenna
a while ago. Cutting two diagonal opposite corners yields circular polarization.

The two diagonals differ 90° in phase, the longer diagonal +45°
(so the antenna is inductive), the shorter -45° (so the antenna is capacitive).

+45 – (-45) = +90°, exactly what is needed for circular polarization (CP).
InmarSat L-band transmits with Right Handed Circular Polarization (RHCP).

This posting attempts to illustrate an utter simple patch antenna built
(in 20 minutes).

Dimensions of 9A4QV’s L-band patch antenna are:

Reflector Size: 170 x 170 mm
Patch Size: 98 x 98 mm
Corner Trim: 21 mm from top right and bottom left corners
Coax Connection (Probe): 25 mm from bottom edge
Height of patch from reflector: 7 mm

Step 1. My reflector is made from 1mm thick aluminium sheet.
The patch is made from 0.5mm thick double sided FR4 PCB. I had some spacers and
cut one with a saw to 7 mm length. Anything else will do, as long as reflector and patch
are ~7 mm separated and electrically connected (the impedance Z = 0 Ω in the middle!).

I chose a N-type chassis connector instead of SMA, but your mileage may vary.
See picture below (click on it to enlarge in a new tab).


Step 2. Create correct dimensions with a caliper, drill holes accordingly . . .
and . . . ready ! See picture below (click on it to enlarge in a new tab)
Note: Fingerprints on the patch element are not mine ; -)

Side view depicted below (click on it to enlarge in a new tab).

Finally, one picture to summarize all (again, click on it to enlarge in a new tab : -)
Yes, I know I made a typo, it has to be corneRs instead of cornes ; -(

August 2nd, 2017

Possible Dutch APRS Outernet first !

Outernet is active for some years now and attempts to realize specific goals :

Afbeeldingsresultaat voor outernet wiki

To provide information without censorship for educational, emergency purposes,
‘news, civil information, commodity prices, weather, construction plans for open source
farm machinery’, and other types of information.
Providing access to ‘courseware’, which includes textbooks, videos, and software.

Basically Outernet is a ‘broadcast datastream’ via the geostationary L-band Inmarsat infrastructure,
using very little bandwidth resulting in around 2.1 kbit/s downstream, and having global coverage.

From what I understood Outernet initially broadcasted from Ku-band satellites (like HotBird)
but changed to L-band. With this change a whole new community emerged, building L-band (1542 MHz)
antennas and receivers dominantly based on cheap RTL-SDR dongles.

Although the Outernet philosophy attempts to present itself as low level and
‘open’ as possible, the core files to decode the transmitted data are proprietary (to some extend)
and/or not public domain.

The latter urged Daniel Estévez EA4GPZ to reverse engineer the Outernet protocol.
Last year he gave a lecture at 33C3 in Germany on this topic (click here to watch his presentation).

Somewhat later/almost simultaneously Pascal Brisset F4DAV ‘hacked’ the Outernet protocol
(with assistance from the information provided by EA4GPZ ?) from a DVB-S perspective.

Having read a lot of information on the internet about Outernet today, Outernet advocates a
nice mission but I doubt it’ll have an added value, besides being a nice and nerdy technical project.

An illustrating posting can be read here.

Considering the latter, a nice technical project with ‘no use’, it is still fun to play with it, so I did.

The HAM-radio community seduced the Outernet CEO to forward APRS packets.

It took me some fiddling to understand how to route APRS-packets to Outernet but after
half an hour I read my first transmission relayed by Outernet!

Below a screenshot as proof (being the first Dutch HAM to have Outernet relayed APRS-packets ? ) !
(and yes, there is a wireless component involved, a 5.6 GHz HAMnet link !)