Triax TDS80 focal point/arm length

[cj955]

Hi,

I have a TDS80A that I have found to have a focal point some way in from where it "should" be - very few LNBs have long enough necks to actually use it properly as most are pushed as far in as they'll go before hitting the sweet spot by some distance.

Wondering if anyone has seen similar and/or if mine just has an oddly long arm? Not wanted to try shortening it yet to avoid the LNB being too low.

cj955

 

[jeallen01]

Incorrect length feedarms supplied by various brands have been reported more than once - though I think this may be the first time that particular dish has been mentioned in this context!

But, given that the dish-end fittings of the feedarms of the dishes from some manufacturers are quite similar across their small to medium ranges, it comes as no real surprise for that dish. Limited neck-position adjustment of LNB's is however also a fairly common issue, and I've encountered that problem myself.

FWIW, I think that @Trust and @RimaNTSS know a lot more about how to work out whether feedarm lengths are correct or not - so maybe one of those members can help you on this issue??

 

[a33]

Welcome to the forum, @cj955 !

Indeed, also a specific version of the Triax 60 (or was it called the Triax 65?) had too long arms.

Determining the exact location of the focal point is doable, but needs some precise measuring of the input parameters for the parabolic calculations.
Are you willing to do that?
The measurements must carefully EXclude the rim of the dish,
it must be ascertained the the dish has (and is meant to have) a flat dish face (otherwise additional measures are needed),
and for instance the depth of the dish has to be measured preferably to a millimeter precise (or even better).

I have often seen people drop out when they see what an elaborate piece of work it is, to do these measurements.
My personal experience with it: I've looked up to the task, and I postponed it at first; but once I was doing it (systematically following the steps), it was not so hard anymore.
So that is why I ask: Are you willing to do that? Or do you just want some 'rough' answer to your question?

Greetz,
A33

 

[cj955]

Hi,

Thanks both for the replies.

I was hoping initially there'd be a "yes, I've seen this with a few of these" but maybe not! I'm not sure I have the kit to measure the dish with enough accuracy for the calculations.

I've spent some time outside today with it and a couple of LNBFs to play around; from what I can test empirically it seems to vary by transponder frequency.

The dish was initially in use for EUMETCast before I replaced it; this sits on 11263 H and V on Eutelsat 10B (as of Monday...). To hand I had a cheap SLx grey single and a current version Inverto BU PLL single. With this on either pol the SLx is far too short & the Inverto works best pressed as far in as it'll go (shame it's a lot shorter than the DRO version). When I tried this some time ago a Labgear quad (the £15 PLL one in Screwfix currently) actually had some travel left on the neck when it peaked with a higher C/N.

Tried the same with a couple transponders on Hotbird 13... namely 10727 H and 11373 H. On the former, there was a *lot* less difference in the focus distance and almost seemed to peak not quite pressed all the way in on the Inverto, with the SLx being a little behind still but by less. On the latter, very similar to how it behaves on E10B's 11263.

Is 500-600 MHz enough to change the phase centre of the feed that considerably?! I wouldn't have expected so. Maybe I should have tried a couple others - but that was enough fiddling as it was!

Also tried cable tie-ing the holder atop the arm and adjusting but this peaked at about exactly the same or slightly lower even after some repeated adjustment.

cj955

 

[a33]

I was hoping initially there'd be a "yes, I've seen this with a few of these" but maybe not! I'm not sure I have the kit to measure the dish with enough accuracy for the calculations.

So it seems you are a kind of pioneer, regarding the focal point location of the Triax TDS80!

To measure the depth value of the dish to a millimeter or half a millimeter, I would recommend a caliper. Otherwise, I guess you'd get an accuracy of about 1-2 millimeter.
What we could do, when you don't have a caliper, is do several depth measurements along the vertical profile of the dish, and see if those measurements lead to about the same focal distance (and thus to about the same position of focal point).
When you do the measuring, I'd do the calculations for you!

All this, assuming the TD80 has a flat dish face (of the paraboloid working area of the dish).

Greetz,
A33

 

[Channel Hopper]

Hi,

I have a TDS80A that I have found to have a focal point some way in from where it "should" be - very few LNBs have long enough necks to actually use it properly as most are pushed as far in as they'll go before hitting the sweet spot by some distance.

Wondering if anyone has seen similar and/or if mine just has an oddly long arm? Not wanted to try shortening it yet to avoid the LNB being too low.

cj955

Incorrect length feedarms supplied by various brands have been reported more than once - though I think this may be the first time that particular dish has been mentioned in this context!

I would, in the first instance ask your local official Triax supplier to confirm the length of the feedarm that is supposed to be delivered with the steel reflector, perhaps opening a box to measure while you are on the call.

 

[jeallen01]

Having only this morning looked at specs and images of the TS80 range, the feedarm-to-dish mounting and the elevation adjustment arrangements look very similar to those of the much earlier versions (of which, over time, I unfortunately acquired 3 different units - TD65, TD75 & TD110!) - and which garnered much (justified!) critiscm about the problems in setting them up.

In that context, the feedarm-to-dish mounting arrangement is a "pig" as, if not done correctly (and that, IMO, means replacing the plastic plugs holding the arm in place by bolts & nuts!) that can allow the arm & LNB to swivel slightly but noticeably up and down - which could mean that the LNB can be in an incorrect position w.r.t. the correct focus point for another reason apart from incorrect arm length.

That's another reason why I fitted bracing brackets between the edges of the dish and the feedarm just in front of the LNB clamp.

 

[Channel Hopper]

This particular Triax has a fixed boom assembly

https://www.alltrade.co.uk/public/product/120508-img-1.medium.jpg

 

[jeallen01]

This particular Triax has a fixed boom assembly

https://www.alltrade.co.uk/public/product/120508-img-1.medium.jpg

Have Triax now dispensed with the plastic plugs that you had to push into the arm to secure it vertically? If so then that's my error (from looking on a small screen) and apologies (but nevertheless the bolt & nut mod is still valid for the earlier versions).

 

[a33]

@jeallen01 :
Here is a pdf on the "new" (2019) triax dishes:

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwjVhbKFrLaAAxUngP0HHeO3AfoQFnoECBUQAQ&url=https%3A%2F%2Fpimapi.triax.com%2FPerfion%2FFile.aspx%3Fid%3D58f43574-25cf-45f0-b480-2b6354375871&usg=AOvVaw2AvPwxG21Nvtji6__B7ibt&opi=89978449

NB Do you have indeed a TD110, reported measures 105 x 100 cm? (As opposed to TD115, which I have, 110.9 x 99.6.)
I always wondered what the real measures of that dish are, as 105x100 does not give offset angle of ~26 degrees. Or would the dish not have a flat dish-face?
Maybe if you are willing to measure and investigate, we could open a new topic.

Greetz,
A33

 

[jeallen01]

Thanks for the link, except that that is for the TD, and not TDS & TDA, series of dishes (and there is not a TD100 listed in the TDS & A series). However, if the arm fixing is the same for the two series, then I can better see the revised method of arm fixture and it does look more secure.

As far as my own large TD is concerned, AFAIK that is a TD110, not TD115, as that is how it was advertised by the private seller from whom I bought it and I think that I did measure and confirm the dimensions after I bought it. However, because of my disabilities now, I am in no position to recheck those as I can hardly walk down, let alone climb up, to it to make them!

 

[a33]

OK, thanks!

 

[Beli1910]

This Triax antenna from the link is a new type, which is made in Turkey, also a Falcom copy - both are manufactured by the same company. The new type of antennas with this logo are not even close to the quality of the production of the old Danish Triax ones. I think I have the measurements somewhere from my Triax 110 antenna. I will take a look and write the exact measurements tomorrow. The best quality Triax antenna for me was the Triax 88 old model.

 

[a33]

I think I have the measurements somewhere from my Triax 110 antenna.

OK, nice!
If it is indeed about 105 x 100 cm (working area of the dish), I'd be curious if the dish face is flat (also working area of the dish, so without the dish rim).
Because a flat dish face, and 105 x 100, would mean an offset angle of just 17.75 degree, whereas Triax reported (about) 26 degrees for its dishes, IIRC.

greetz,
A33

 

[cj955]

Oops! I wrote out a reply but forgot to send it so it was lost when I turned off the PC

The type a33 links to is indeed the right one; the new TD series TDA/TDS are just alu/steel sub-models. The design has totally changed from the old TDS and the feedarm clicks into place and then the back brace is clamped around it using a bolt through that just above the arm (to allow for cables) - honestly don't like that, tighten it down enough that the arm is actually solid and the thing just starts to bend the arm out of shape. And no washers included so unless you add some it starts deforming... But no weird plastic clip thingy. The El adjust is probably a bit better but still has loads of slop and you have to spend so much time fiddling to keep the Az set whilst adjusting it...

I actually contacted Triax UK; they confirmed that the dish face should be flat laid on its face. But they didn't have access to design drawings (!!!) so couldn't confirm things like the arm length. My dish came from CPC Farnell; not exactly a specialist so not sure if it's even worth the phonecall...

I do have a (good Mitutoyo) digital caliper available but I don't think I have an accurate straight edge big enough to reach over the dish. Or I could give it a go with the strings method I guess!

It performs well enough for TV use - I was watching the Antares/Cygnus launch last night off Hotbird 13 and that sits over 16 dB C/N as measured on a TBS 5930. More just chasing every last bit out where I was using it for EUMETCast.

I think a bigger dish might be the real solution. It was replaced for said use with a Channel Master 84E; I have actually just been experimenting with that as the performance was.... literally no better (other than in the wind - that thing is rock solid) with its supplied feed and an Inverto Pro C120. Despite being elliptical I've got nearly a whole 1 dB improvement by chucking on first a Labgear quad and later an extremely cheap Maclean MCTV-252 from Bezos Books last night (I had to borrow the Labgear from the 28.2E zone 1 dish...) which is very welcome for the nasty 16-APSK 2/3 (well, one is VCM with 8-PSK too) signals and their 9.3 dB minimum... I get up to 13.1 dB ish C/N on the stronger of the two transponders and 11.9 dB on the weaker now. Not great rain margin.

I suppose at least London is in the area that got marginally stronger with the Eutelsat 10B switch; it seems to have a narrower footprint like Hotbird 13G and the guys up in Scotland have lost a good bit.

cj955

 

[jeallen01]

@cj955

Pity the problems didn't show up earlier as CPC Farnell are very good at accepting returns if something is faulty, or simply not suitable.

PS: They are not the best place from which to buy sat dishes in the first place!

 

[a33]

I actually contacted Triax UK; they confirmed that the dish face should be flat laid on its face.

BTW. If the dish rim has a flat face, it still must be checked that the dish rim itself has the same 'height' at top - bottom - left - right. So that the parabolic working area of the dish also has a flat dish face.
(The TD115 is like this. And measured height and width give offset angle 26.1 degree. See e.g. here: Triax TD115 met te korte arm? )

I do have a (good Mitutoyo) digital caliper available but I don't think I have an accurate straight edge big enough to reach over the dish. Or I could give it a go with the strings method I guess!

1 mm off in depth measure would probably give focal length difference of about 7 mm off, I just checked. But we can surely try!

A very extensive "how-to" for the measurement of height, width, and depth-at-the-center of a paraboloid dish is here in the post by @strannik: Measurement of parabolic antennas - Satellites Community
(He uses strings, to measure depth from!)
(You can also find his calculation program there. Excellent stuff. Though I have a slightly different opinion where to aim the LNB.)

I think a bigger dish might be the real solution.

That is always a solution. But it is also good, to try to get the maximum out of your existing antenna!

Greetz,
A33

 

[Channel Hopper]

BTW. If the dish rim has a flat face, it still must be checked that the dish rim itself has the same 'height' at top - bottom - left - right. So that the parabolic working area of the dish also has a flat dish face

As per my comment on another thread not long ago, placing the dish against a patio window should give a clue on any warpage of the reflector.

 

[a33]

I think I have the measurements somewhere from my Triax 110 antenna. I will take a look and write the exact measurements tomorrow.

I'd still be interested to see those measures!

If it is indeed about 105 x 100 cm (working area of the dish), I'd be curious if the dish face is flat (also working area of the dish, so without the dish rim).
Because a flat dish face, and 105 x 100, would mean an offset angle of just 17.75 degree, whereas Triax reported (about) 26 degrees for its dishes, IIRC.

Greetz,
A33

 

[jeallen01]

I'd still be interested to see those measures!
Greetz,
A33

FWIW, from the TRIAX installers guide