Episode 2 of Improving DVB-T Magmount Antenna.


#1

I have been using the DVB-T’s magmount whip antenna after trimming its removeable part to 67mm length as shown in the image 1.

Recently I opened the base of the magmount antenna (images 2, 3 & 4). I noted that there is about 15 mm of vertical metal part between screwable whip and feed coax.

This hidden vertical metal is part of antenna, and hence antenna length should be measured from the bottom of this part where coax is soldered to it. Since this part is 15 mm long in my antenna, my magmount’s length was actually 67 mm + 15 mm = 82 mm. I therefore chopped off another 15 mm from the whip, making the length of removeable part 52 mm instead of 67 mm. See image 5. I noticed about 5% to 10% improvement.

Image 1 of 5

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Images 2, 3 & 4

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Image 5

Things to be tried:

  1. Remove black plastic circular adhessive cover from bottom of antenna, and check electrical continuity between exposed metal base plate & the outer metallic part of MCX connector at other end of antenna’s coax.

  2. Place exposed base on a metallic can and check if direct contact gives any improvement.


Same Setup/location total different results
Three Easy DIY Antennas for Beginners
#2

So, if my Maths is correct, it would be about 75mm for 978Mhz UAT.


#3

Yes, you are right. Please see sketch below.

In the sketches in the first post & this post, the “length inside base” has been taken as 15 mm. This is based on measurements of my stcock antenna. It may be somewhat different for different manufacturer.

Generally in a 1/4 wavelength mono-pole/groundplane antenna, few mm error in length will not make big difference. However if you want exact length, please check “length inside base” by opening base of your whip antenna.


#4

On mine the little plastic cap at the tip of the antenna just pulls off, so I did that before cutting then put it back on the shortened antenna.

Also these might be made of hard steel “piano wire”, which will damage most wire cutters. Use a grinding disc or wheel instead to cut it.


#5

I found that the can I was mounting my antenna on was coated in a thin film of clear material, probably to reduce it corroding. I used a small piece of sandpaper to rough the surface of the can under the antenna to give a really good contact and of course checked the continuity after doing this.


#6

What was the result of direct electrical contact?
Did you notice any improvement in performance, i.e. any increase in range, number of planes, and message rate?


#7

I’ve only had the can on place for 48 hours and the whole system up 7 days so it’s a little early to tell, but initial thoughts are the blue has darkened slightly.

Currently the unit is in the highest part of a loft space, but is blocked to the west by a brick chimney and east by a neighbouring house which is quite obvious by the coverage graph. I will be making further adjustments over the coming weeks to try to improve this as this is the flightpath for London Stansted and Luton airports.


#8

@AdrianHales:
The biggest part of improvement comes from chopping the whip to proper length. Adding a Can also improves, but not to the degree the chopping off the whip improves. Hope you have already chopped off the whip.