Electronics > RF, Microwave, Ham Radio

Help making DIY balun from DBT LogPeriodic Antenna to 490-700mHz preamp

(1/6) > >>

Paul Price:
How do I make a balun?

I live in Copenhagen Denmark and want to receive DBT TV from Malmo Sweden and I have a .3 meter long indoor tabletop  log-periodic antenna(mounted outside on the back porch), but all Swedish TV signals are very weak, and even with an adj. gain antenna preamp (up to 30db adj. but has no LTE filter), I can only see a few hours of one Malmo TV station per week.

I have found that no satellite TV or  local TV cable-tv services do not offer but only 3 of the 8 possible Swedish freeview TV stations, so that is not a good option.

The local Copenhagen transponders give a strong signal and I have no trouble receiving them, but their broadcast is of little interest to me!  A coat-hanger attached to coke cans works well for local signals!

Due to restrictions on what I can attach to my back porch of my apt. building, I cannot mount anything but a satellite dish or a small non-obtrusive antenna on my tiny back porch.

It may be that I am only receiving one strong Swedish TV station, but only occasionally, that is about 30-miles away because of atmospheric skip and the situation seems hopeless, but I have not yet explored all my options

The antenna has no balun and when I tried inserting a standard TV 300-ohm line to 75 ohm balun to connect to the preamp the signal was gone completely.

I am connecting the front of the antenna directly to 75-ohm coax and making connections by tightly winding many turns of silver plated 32AVG stripped wire-wrap wire around the front most element pair, near the center boom (which is plastic).

Surprisingly, when I bought a mushroom Yagi antenna of 4-ft length, there was no reception of any but the local Dan channels, so this desktop antenna is the best solution so far for what  I have to work with.

Reception was much better before LTE/4G service started many years ago. I have found there is now a 20-DB DVB antenna preamp available with LTE/4G filter, but I haven't tried this option yet.

If I was to purchase a LTE/4G filter I would still need to build a balun to connect coax directly to the antenna, would inserting a filter before the preamp help with such a weak signal?

I have some discarded TV/Internet routers that multiplex TV and broadband internet on the same 75-ohm coax cable, and I see within the RF shielded box connecting directly to the 75-ohm F-connector a 3-4mm ext. diam toroidial core used to match the cable to the broadband internet processing chip section of the router.

Please, can some rf guru's help me, give me some help in using this small ferrite core to wind a balun to interface the antenna to the 75-ohm unbalanced antenna preamp?


BTW, I am very experienced in electronics design and have the ability make my own PCB for circuits, but know only so little about RF.

BigBoss:
Don't bother yourself by designing/realizing such balun.
Use this ( or similar) balun from Mini-Circuits and just after place an Low Noise Amplifier with a single transistor for that band then place them all
on a single PCB by taking care of RF principles.
https://www.minicircuits.com/WebStore/dashboard.html?model=TC4-122-75X%2B

wasedadoc:
For the same length a Yagi for the correct channel group has significantly more gain than a log periodic.  Get as much antenna gain as you can before adding an amplifier.

Paul Price:
As I've mentioned, I have already tried using a standard legacy 300-ohm line to 75-ohm coax balun and the result was a complete loss of any Swedish channels.  This failure to work could be due to a very poor quality balun or (more likely IMHO) a mismatch?
The Mini-Circuits is a 4:1 balun which expects to be connected to a feed point on a 300-ohm dipole.

The Mini-Circuits balun you recommend has very good specs(read the spec sheet), but I expect I would still see poor results due to the way I position a feed  on the very front-most dipole elements of the log-periodic antenna. I expect this is creating an impedance mismatch.

Consider this:
The elements on my log periodic antenna are connecting alternating horizontal sides along the plastic boom over the length of the antenna.
The top and bottom elements are crisscrossed and are separated vertically by the width of 1-in plastic boom. The separate planes of the elements cause elements to overlap each other after each element crisscrosses in horiz. position.
 
Where on front element would give the optimal impedance match to the balun? My feed point now is at the point where the element is attached to the plastic boom and then starts to  cross over to the next element behind.

Paul Price:
wasedadoc,

As I've mentioned in my opening post, a yagi antenna could would not be permitted to be mounted on my back porch because it would be so long that it would be an obvious eyesore.

When I bought a mushroom triple boom Yagi antenna of 5-ft length, there was no reception of any Swedish channels, no channels but the local Dan channels were received, so this desktop antenna is the best solution so far for what  I have to work with.

Navigation

[0] Message Index

[#] Next page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod