Interesting Kathrein 70cm train antenna design

[wkb]

I got my hands on an interesting Kathrein 70cm antenna designed for use on trains. Originally it was equipped with a funky proprietary Kathrein coax connector. This stainless steel connector was equipped with a spring-loaded silver-plated pin inside a silver-plated brass tube.
Very nice engineering, my guess is that this was done to ensure continuous contact in high vibration situations(?).

Given that I did not fancy spending like EUR 15 on a suitable plug, I rebuilt the antenna using an N connector. Versions with N connector are also produced by Kathrein btw.  It is built like a tank (well.. train I suppose): Nylon cap, diecast aluminium, stainless steel etc.

Would be interesting if someone could explain the design...  Or model it in something like EZNEC. Note that it requires a groundplane (read: train) to work properly.

[Theboel]

Sorry what the enclosure material ? Is that stainless steel ?

[wkb]

Sorry what the enclosure material ? Is that stainless steel ?

The black stuff is Nylon.

[T3sl4co1l]

Yabbut it doesn't look black, it looks metallic, and shiny around the edge..?

If it is nylon after all, then it'll have little effect on the fields, and you'll have something like a patch antenna there.  Curious that they have a tuning "foot" on it.  I would expect the radiation pattern is typical of any 1/4 wave whip type antenna, or patch or whatever.  Bandwidth will be on the low side.

Tim

[wkb]

Yabbut it doesn't look black, it looks metallic, and shiny around the edge..?

If it is nylon after all, then it'll have little effect on the fields, and you'll have something like a patch antenna there.  Curious that they have a tuning "foot" on it.  I would expect the radiation pattern is typical of any 1/4 wave whip type antenna, or patch or whatever.  Bandwidth will be on the low side.

Tim

Ah, misunderstanding on my part I think: the shiny stuff that the N connector is mounted on is die-cast aluminium.

[T3sl4co1l]

Yabbut it doesn't look black, it looks metallic, and shiny around the edge..?

If it is nylon after all, then it'll have little effect on the fields, and you'll have something like a patch antenna there.  Curious that they have a tuning "foot" on it.  I would expect the radiation pattern is typical of any 1/4 wave whip type antenna, or patch or whatever.  Bandwidth will be on the low side.

Tim

Ah, misunderstanding on my part I think: the shiny stuff that the N connector is mounted onnis die-cast aluminium.

Yeah that's all solid, actually it looks like sand cast not die cast(!), but I mean the outer shell in the first two pictures.

Tim

[wkb]

Yabbut it doesn't look black, it looks metallic, and shiny around the edge..?

If it is nylon after all, then it'll have little effect on the fields, and you'll have something like a patch antenna there.  Curious that they have a tuning "foot" on it.  I would expect the radiation pattern is typical of any 1/4 wave whip type antenna, or patch or whatever.  Bandwidth will be on the low side.

Tim

Ok, we are refering to the weather radome after all. That is black nylon.

If you feed the Kathrein part# to Google it will give you a catalogue with some limited specs.

Ah, misunderstanding on my part I think: the shiny stuff that the N connector is mounted onnis die-cast aluminium.

Yeah that's all solid, actually it looks like sand cast not die cast(!), but I mean the outer shell in the first two pictures.

Tim

[cdev]

Very high efficiency UHF loop using gamma match?

[rfeecs]

That antenna is described here
http://www.kathrein.pl/down/BasicAntenna.pdf
On page 15:

A further possibility of shortening the antenna length can be found by using a top loading capacitance
which artificially lengthens the radiating element. With this method it is possible to construct very flat
antennas. Figure 34 shows a 70-cm-Band antenna with a vertical length of 70mm. The resulting gain
of these shortened antennas depends on the degree of shortening whereby the thickness of the radiating
element plays an important role.

It is a shortened quarter wave antenna that is top loaded.  Page 16 talks about train antennas having a shorting rod for safety purposes.  This one seems to have a shorting rod that is shorter than a quarter wave as well.

The antenna is also on page 12 of this catalog:
https://www.kathrein.de/fileadmin/media/druckschriften/99812350.pdf
It has a gain of zero relative to a quarter wave antenna and has vertical polarization.  So it makes sense that is would be a shortened quarter wave antenna.

[TheSteve]

Any chance you can sweep the antenna and post a plot?

I totally get the capacitive hat but I'd like more theory on the tuning of the shorting bar on the left and what I assume is a matching network on the right.

[rfeecs]

This patent seems to describe a similar antenna:
http://www.google.com/patents/US6917341

[T3sl4co1l]

You could think of it as a short loop antenna, with some capacitive loading on the gap.  The gamma match is equivalent to a lower tap, or a coupling winding, and the capacitive hat gives some voltage radiation but isn't at the highest voltage point so it won't be quite as effective, but that's all just a matter of matching.

Although the structure is rather complicated, the physical size is too small to have a radiation pattern anything different from a traditional monopole or loop.

I would guess the bandwidth is on the order of 10% or less, and there will probably be secondary peaks at much higher frequencies, where the length of the gamma match or tuning leg become characteristic.  Probably at 1/4 to 1/2 wave long and multiples thereof.  At those frequencies, the capacity hat will act like a ground plane.  And, of course, you'll have more complicated radiations patterns, depending on the phasing (electrically and by physical distance) between those elements.

Tim

[wkb]

Any chance you can sweep the antenna and post a plot?

I totally get the capacitive hat but I'd like more theory on the tuning of the shorting bar on the left and what I assume is a matching network on the right.

I can hook it up to my SA with a directional coupler.  Need to find some metal plate for a groundplane and mount it on a pole so that I can put it in the garden in free space. Will take a while

[wkb]

Any chance you can sweep the antenna and post a plot?

I totally get the capacitive hat but I'd like more theory on the tuning of the shorting bar on the left and what I assume is a matching network on the right.

I can hook it up to my SA with a directional coupler.  Need to find some metal plate for a groundplane and mount it on a pole so that I can put it in the garden in free space. Will take a while

Well.. it has taken a while..

Here is the sweep.  Lacking a train I mounted the antenna on an approx 60cm diameter stainless steel plate. Some improvisatiion allowed it to be mounted on a 3.75meter high GFK pole (military surplus pole for camouflage netting).

Wilko