Teardown: Kathrein GSM900 base station sector antenna - Page 1
Forum section: electronics / rf-microwave

author: wkb
author_rank: Super Contributor
timestamp: February 27, 2016, 10:45:03 pm
content: Today I visited a ham-fest to gain some much needed (....) parts & 'stuff'. One of the things on my shopping list was some semi-rigid coax.As it turned out, no loose semi-rigid was to be found.What I did find and what had ample semi-rigid was a Kathrein GSM900 cell base station antenna. Which changed hands for the princely sum of EUR 5. Me dragging it out of the building gave rise to to more than few raised eyebrowsAfter muttering some niceties because it onlyjustfitted in the car I took it home.Picture series shows the teardown of this antenna. This particular version has separate 7/16 connectors for connection to the horizontally and vertically polarised antenna. Construction is very solid Deutsche GrÜndlichkeit: stainless steel bolts, silvered connectors, tons of pop rivets to obtain a very rigid construction etc. The Kathrein catalog promises 15 years of maintenance free operation. As can be seen extensive lengths of semi-rigid, or more exact Huber & Suhner Sucoform 141. The cables are connected via soldered-on 'junction boxes'. The white blocks are plastic spacers.

author: wkb
author_rank: Super Contributor
timestamp: February 27, 2016, 10:49:23 pm
content: More pictures

author: wkb
author_rank: Super Contributor
timestamp: February 27, 2016, 10:50:47 pm
content: Yet more pictures

author: wkb
author_rank: Super Contributor
timestamp: February 27, 2016, 10:52:10 pm
content: And a bit more pictures, one with part of the wiring

author: wkb
author_rank: Super Contributor
timestamp: February 27, 2016, 10:59:42 pm
content: More

author: wkb
author_rank: Super Contributor
timestamp: February 27, 2016, 11:02:04 pm
content: more

author: wkb
author_rank: Super Contributor
timestamp: February 27, 2016, 11:03:16 pm
content: Coax plumbing

author: wkb
author_rank: Super Contributor
timestamp: February 27, 2016, 11:06:39 pm
content: more plumbing

author: wkb
author_rank: Super Contributor
timestamp: February 27, 2016, 11:10:05 pm
content: The harvest: a considerable amount of semi-rigid

author: HAL-42b
author_rank: Frequent Contributor
timestamp: February 27, 2016, 11:20:32 pm
content: So both vertical and horizontal polarization. Didn't know that.

author: wkb
author_rank: Super Contributor
timestamp: February 27, 2016, 11:28:48 pm
content: Quote from: HAL-42b on February 27, 2016, 11:20:32 pmWell, some antennas at the ham-fest had both H and V, some only had V ( I think it was).I grabbed the dual H/V given that it contains more coax

author: wkb
author_rank: Super Contributor
timestamp: February 28, 2016, 07:00:08 pm
content: Some detail pictures of the individual dipole elements. They are directly fed with 75Ohm Sucoform semi-rigid. Note everything is at DC ground. The elements are made of solid copper, judging from the edges where they were cut. The copper is tin(?)-plated which makes for the non-copper appearance.Aside: I found 3 (at least) impedances of the Sucoform were used: 75 Ohms, 100 Ohms, and 50 Ohms. Not all of it is clearly marked unfortunately.

author: SeanB
author_rank: Super Contributor
timestamp: February 28, 2016, 07:55:27 pm
content: So that is what was inside the old ones on the roof. they were replaced last year with new ones, as the carrier is migrating away from GSM900, so there are some GSM1800 and UTMS steerable antennas there now. All connected with corrugated rigid line down to the base station on ground level.I think the old Kathrein ones were sold off to another carrier building out a network, which uses GSM900 in low density areas for the improved coverage, and that most of the customers in those areas also are low bandwidth users, mostly using voice and SMS, or GPRS data.

author: PE1RKI
author_rank: Regular Contributor
timestamp: February 28, 2016, 09:48:49 pm
content: i dismantled also one, i managed to snag this one from many 100's of these that all got scrapped. all unused, from unopened new boxes.luckaly all the semirigid inside was 50 ohm. inside was very different to yours.here is a link to my flickr album of the antennahttps://www.flickr.com/photos/pe1rki/albums/72157643213030984it even had tuning for elevation or something and lasercut brass airlines with different kind of impedances for stacking the antenna's i guess.

author: daqq
author_rank: Super Contributor
timestamp: February 29, 2016, 09:52:01 am
content: Thanks for the pics!

author: wkb
author_rank: Super Contributor
timestamp: February 29, 2016, 03:28:01 pm
content: Quote from: PE1RKI on February 28, 2016, 09:48:49 pmVery different indeed! It contained sufficient 50R semirigid for what I want to build (RLB to go onto the SA). Would have been nicer to have all 50R but hey..Wilko

author: PE1RKI
author_rank: Regular Contributor
timestamp: February 29, 2016, 06:23:06 pm
content: it was all habia cable flexiform 402 nm hfi 100.50 ohm according to the datasheet.i dont know how much you need but i can miss a piece of 40cm

author: wkb
author_rank: Super Contributor
timestamp: March 01, 2016, 11:44:14 am
content: Quote from: PE1RKI on February 29, 2016, 06:23:06 pmThanks for the offer! I have 2 pieces approx 60cm each of what should be 50R. After all they were connected to the 7-16 connectors.Not yet sure what I will do with the 75 and 100 ohms Sucoform. tweedehands.nl comes to mindmaybe someone can make good use of it.

author: wkb
author_rank: Super Contributor
timestamp: March 01, 2016, 06:48:28 pm
content: Today I decided I want to see if I can use a chopped-off part on 23cm. So I shortened the aluminium base. Not the dipole elements yet. The chopping resulted in picture #1 below.After remounting 2 dipole elements I noticed that the vertical and horizontal dipoles have different dimensions, see picture #2Most likely that is because the elements now mounted on the base protrude over the edge of the base. So Kathrein have tuned this carefully (obviously.., this is a leading aerial manufacturer after all!).So now I have to figure out how to join the 2 dipoles and ensure a 50R coax connection (using that interesting set of junction boxes and different impedance semi-rigid pieces). Now how did that work again, coax impedance transformation..

author: PE1RKI
author_rank: Regular Contributor
timestamp: March 02, 2016, 01:44:08 pm
content: you can use the 75 ohm coax now.and make it like the wilkinson combiner but without the resistor.2 pieces of 1/4 lambda coax splitting of to each antenna.between antenna and the 75 ohm coax you can use 50 ohm again if the 75 ohm is to short.it must be 70.7 ohm but i guess it will work ok with 75 ohm to.

author: wkb
author_rank: Super Contributor
timestamp: March 02, 2016, 03:47:42 pm
content: Like the one in the picture? The dipole elements still have theoriginal 75R coax in sufficient length attached to them for 1/4lamba@23cm I think

author: PE1RKI
author_rank: Regular Contributor
timestamp: March 02, 2016, 03:49:49 pm
content: yes!the 75 ohm lines are also shortened by the velocity factor.

author: wkb
author_rank: Super Contributor
timestamp: March 02, 2016, 04:27:36 pm
content: Quote from: PE1RKI on March 02, 2016, 03:49:49 pmSucoform 141-75 datasheet tells me the VF is 71% ehh 0,71 so that helpseven more. The antenna in its original incarnation had these nice littlejunction box so I can re-use those.To what extent is the distance between the dipole elements of influence?Sofar have not found too much on how these antennas are designed.

author: wkb
author_rank: Super Contributor
timestamp: March 02, 2016, 04:47:03 pm
content: ,just spent some time staring at pictures of the original coax harness. And noticed that the long type dipoles have different length coax between the junction box and the dipoles forming a pair.Any suggestions?

author: PE1RKI
author_rank: Regular Contributor
timestamp: March 02, 2016, 05:25:51 pm
content: different frequency? maybe transmit and receive frequencies ?the size is so different, it doesnt make sense.best step forward is to put the antenna's individually on a network analyzer.

author: wkb
author_rank: Super Contributor
timestamp: March 02, 2016, 06:16:46 pm
content: Quote from: PE1RKI on March 02, 2016, 05:25:51 pmWell, the short dipoles had symmetrical coax lengths to the junction box. I imaginethe H and V dipoles are for some kind of polarisation diversity setup(?). That I can understand.But the long dipoles with different cabling. no. ENOCLUEI don't own a network analyser. And (uhm..) I did a rather thorough job in dismantling the thing.Kept only two long dipoles with cable still attached.There is quite a bit more to this beast than I had imagined

author: PE1RKI
author_rank: Regular Contributor
timestamp: March 02, 2016, 06:54:55 pm
content: if you need a little antenna for 23cm then you can also use the 8 antenna.about 15 years ago i made a little guide for this little antennahttp://pe1rki.com/quad.htmlvery crusty pics, i have very little free time to update my website, i'm aware it has tons of mistakes.i made one for 13cm as well and my 500mw tv picture was received 40km's away.tx and rx were 25meter and 35meter above ground level though.

author: wkb
author_rank: Super Contributor
timestamp: March 02, 2016, 06:59:58 pm
content: Quote from: PE1RKI on March 02, 2016, 06:54:55 pmAh yes, I once built one like that for WiFi. Worked like yours: very nicely indeed!The whole idea is to play with this GSM thing "because I can". Very practical in use... ah, maybe not.

author: PE1RKI
author_rank: Regular Contributor
timestamp: March 02, 2016, 07:03:59 pm
content: you can truly play with it if you can measure the results.without equipment it is always walking in the dark.

author: wkb
author_rank: Super Contributor
timestamp: March 02, 2016, 07:06:42 pm
content: Quote from: PE1RKI on March 02, 2016, 07:03:59 pmI plan to use a SA with TG and. RLB. Should give me an idea or?

author: PE1RKI
author_rank: Regular Contributor
timestamp: March 02, 2016, 07:11:35 pm
content: yes that works. i never used that myself though.with no reflected signal from the antenna you at least know it radiates, but you dont know in what direction yet.

author: wkb
author_rank: Super Contributor
timestamp: March 02, 2016, 07:55:22 pm
content: Quote from: PE1RKI on March 02, 2016, 07:11:35 pmAt least I know the direction won't be through the alu base plateMeasured the dipoles in the meantime: 14 and 16cm span respectively. 16cm I can match to the 900MHzGSM band. The Kathrein label says 870-960MHz. But the 14cm ??And there is a little problem: 1/4 lambda in coax with VF 0,71 is really short on 23cm. Too short to bridge the distance between 2 dipoles using the original base plate.

author: PE1RKI
author_rank: Regular Contributor
timestamp: March 02, 2016, 08:05:22 pm
content: you can add 50 ohm coax in between the antenna and 75 ohm pieces.i dont get those lengths either.maybe it has to do with the distance above the alu baseplate.

author: wkb
author_rank: Super Contributor
timestamp: March 02, 2016, 08:39:38 pm
content: Quote from: PE1RKI on March 02, 2016, 08:05:22 pmBoth dipole types are approx 5.5cm above the baseplate. The 16cm version slightly protrudes over the alu reinforcement(? for stiffness?) 'rib'.Originally it uses 75R soldered onto the dipoles. Does that matter?

author: PE1RKI
author_rank: Regular Contributor
timestamp: March 02, 2016, 09:27:58 pm
content: yes that may mean that the dipoles are not 50 ohm.then our combining method will not work.

author: wkb
author_rank: Super Contributor
timestamp: March 02, 2016, 10:00:46 pm
content: Quote from: PE1RKI on March 02, 2016, 09:27:58 pmOk. There must be a reason for the interesting mix of 50,75 and 100R coax after all.