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Offline Vansflyer

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Re: DAB Digital Radio (UK ) for the Trophy SE with Forum Discount Offe
« Reply #96 on: April 26, 2019, 06:47:14 PM »
I think the T antenna split may not help, we should meter before and "after". If you can measure the resistance with one and then both aerial connected, this will confirm if the impedance is being reduced.  The sheet wont work unless it is connected to the base of the antenna by magnet or physical contact.
Attached is a ground plane fitted to a plane with a plastic fuselage a mate owns, it's for a transponder but you will get the idea. Doesn't actually need to be that big... Do you have a powered Topbox? In theory you could make the whole bike the ground plane by tagging onto the neutral wire in the topbox. This isn't perfect as it loads the aerial with a better direction angle pointing one way but better than nothing. The issue you will have is connecting that wire neatly from the outside inside... The best answer is a drill and screw the thing on with a proper ground plane inside, must be some s/hand topboxes around?
Lets meet on a Monday evening to discuss  :001:

Offline Coconut

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Re: DAB Digital Radio (UK ) for the Trophy SE with Forum Discount Offe
« Reply #97 on: April 26, 2019, 07:05:46 PM »
:028:  :821:  :169:

Smiles are infectious,and they are FREE,
so if you see someone without one, give them one of yours :-)

Offline Grubloon

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Re: DAB Digital Radio (UK ) for the Trophy SE with Forum Discount Offe
« Reply #98 on: April 28, 2019, 09:45:01 AM »

Have read the comments made by Vansflyer, re the metal plate and top box mounting, my question is what size, thickness and metal type can be used as the ground plane, also if one used the bike as the ground plane, could a wire bolted to a good earth point be suitable.

 :028:

Offline Vansflyer

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Re: DAB Digital Radio (UK ) for the Trophy SE with Forum Discount Offe
« Reply #99 on: April 28, 2019, 11:04:39 AM »
Read the wiki article on ground planes and it explains it quite well. Basically you need a pice of conducting metal in theory 1/4 wave size or more of the frequency you are receiving/ transmitting on.
Look here for an example.
https://lxavionics.co.uk/index.php?route=product/product&product_id=265&search=Ground+plane&description=true
That is quite expensive for what it is, in theory you could use Ali strip on a roll in an X or + shape in the topbox lid. Done this before in a plane and it worked well. You can ground to the bike but it's won't give a balanced view of reception but better than nothing for sure. Hence my comment about trying it connected to the neg lead if you have power in the topbox. Bear in mind the antenna will need a ground connection if it is powered unless it uses the Dab units ground, which it might. The T bar Coconut was using must use the neg of the supplying device as it is "stick on".The windscreen Dab aerials connect to the windscreen surround to make the ground plane, have one in my old car and it works remarkably well. This propagation stuff can get quite complicated....

Offline Grubloon

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Re: DAB Digital Radio (UK ) for the Trophy SE with Forum Discount Offe
« Reply #100 on: April 28, 2019, 02:00:18 PM »
Vansflyer,
This DAB radio malarkey is more complicate than I thought, but I will stick with it, thanks for the info.

 :084:  :187:   

Offline thogland

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Re: DAB Digital Radio (UK ) for the Trophy SE with Forum Discount Offe
« Reply #101 on: April 28, 2019, 05:47:57 PM »
*Originally Posted by Grubloon [+]
Vansflyer,
This DAB radio malarkey is more complicate than I thought, but I will stick with it, thanks for the info.

 :084:  :187:
It's not too bad - as a HAM I picked up enough to figure this stuff out. Looks like DAB broadcasts on about 225Mhz, which is a 1.3m wavelength. Your ground plane needs to "stick out" from the antenna base 1/4 of that length - 33 cm. (Looking at the pictures, it appears that's what the antenna shown is - 1/4 wave.)

Tough to get that much length on the top box, even with the antenna dead-center. But, one way would be to pull the shell off the top case, put wires that long in a "X" corner to corner. The curve doesn't help things, but it's better than not having anything. Maybe make them 34cm instead, to help offset the curve.

More ground plane is better, but two sticking out opposite works. Having the "four corners" of the top box to play with is convenient. If you put it at the front edge of the top box, and ran two diagonally to the rear corners, it'd still be better than nothing.

Another option, if you want to replace your current FM antenna with this DAB one - put it in the original location, then put one ground plane sideways across the nose inside the fairing. Maybe even a second one pointed back towards the battery. Should give decent reception.

(Side-note fun fact: Ever wonder why all car radio antennas are the same length, or used to be? It's because FM radio is in the 3m band, and 75cm is the ideal practical length. My truck has a much shorter antenna - to receive both FM and XM - and uses electronic wizardry to selectively filter and amplify as needed.)

You can get WAY involved in wave propagation theory if you get too deep into this. Enough to know how to find the frequency band (easy to find references online), then your antenna will ideally be the same length. Since in this case putting a 1.3m antenna on the Trophy is clearly not possible, then you halve it - 65cm (a "half wave" antenna). Might be possible, but still tough, so halve it again to 1/4 wave - 33cm. Now this is doable.  :001:  Going shorter than 1/4 wave doesn't work as well without electronic assistance.

Oh, and if you have a 2m or 3m antenna cable, don't cut it or tie it in a tight little bundle - coil it up in a circle. There's interactions between antenna length and cable length, cable length and cable resistance, etc. and coiling it neatly in about a 10cm circle will minimize all of that.

Offline Grubloon

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Re: DAB Digital Radio (UK ) for the Trophy SE with Forum Discount Offe
« Reply #102 on: April 28, 2019, 06:41:37 PM »
So, if I've understood your reply correctly thogland, a car roof acts as a ground plane, and the antenna can be quite small, as on my car like a small sharks fin.

So ideally a top box lid of light steel would do the job, but that is not very practical.

Your note about not shortening the antenna lead was something I knew about, so I always just long looped the lead along the rear of the frame, your note about coiling it into a 10cm circle, is it imperative that this is done, as it would be a bit of a pain to do.

Will do a bit more reading to try and understand this subject a bit more.

 :152: