Need advise on combined antennas

[domu]

I need to combine output from two antennas to send it down
through approx. 50m single coax run to feed two receivers.
ANTENNA 1 is for 1040-1090 (Mode2/ADS-B), ANTENNA 2 is for
118-136 (Airband). I am aware that combining input from two
antennas may be challenging, but since those antennas are
dedicated for their respective bands - I speculate that it
wouldn't be a source of major problems. And just to stay
safe I would add bandpass filter ~1050 on the output from
ANTENNA1. I understand that 50m coax run, plus combiner
and splitter will introduce significant loses to the signal,
hence I would add masthead amplifiers for both antennas.
Below is ASCII diagram of what I hope would work:

                                                            /-- POWER
ANT1 --B/P FILTER--M/H AMP --\                             /-- RCVR1
                              COMBINER---COAX50m---SPLITTER
ANT2 ------------> M/H AMP --/                             \-- RCVR2

I do not have any expertise in this field, this is one time
project, and I doubt if I ever will return to this subject
again. Just need to solve the problem, and want to do it as
good as possible. Therefore, I have a couple of questions:

- would it work ?

- is there a better solution ? (must be single coax)

- what coax could I use (RG-8/U,RG213,LM400,CNT400,9914F) ?

- what amp/splitter/combiner would you recommend ?

- does it matter how those two antennas are far apart ?


Thank you for any helpful answers.

 

[sv1437]

For the coax, select the one which has least attenuation for the frequency range (100 to 1100 MHz, considering both the freq bands). For power splitter/combiners and amplifiers, you can get from **broken link removed** for the frequency range (100 to 1100 MHz).

For the project are you allowed to use custom made parts or you have to build your own splitter or amplifier ?

 

[vfone]

Because you have relative big distance between bands, better than a BPF is to add a diplexer (not duplexer) where the antenna combines.
A diplexer is a combined LPF and a HPF. They have wider rejection bandwidth and also have lower in-band insertion loss than a BPF (or a duplexer which is a combination of BP Filters).

 

[domu]

->sv1437 - as for the coax, I would gladly select any of ~400 type as they
are advertised to have around 13 dB at 1000 MHz (as opposed to RG8/U likes
which, I read, are around 29 dB). The problem is that down here anything
~400 is on average four time more expensive. So, I think, as soon as I can
find some decent amps - I will need to do a bit of math to balance budget
and performance. Regarding parts, I hope to find some good "off the shelf"
components, hence my request for recommendations.

->vfone - yes, I was actually looking around to find suitable diplexer, but
could not find anything that would cover those two specific bands (even
loosely). Could you have any specific recommendation, please ?

 

[ghb]

i would suggest as vfone did a diplexer. because your frequencies are so spread apart this should be easily achievable with lumped elements with minimal <.3 dB insertion loss at highest frequencies. you could make this design and board yourself, if you have the means. you need the diplexer as apposed to a single filter as the impedance of the ant2 in your diagram will most likely not be a RF open at ant1's frequencies. if it is no an rf open you will lose efficiency. this type of combining with diplexers is common.

the spacing of the antennas is important, particularly the impact of the lf on the hf antenna. for minimal impact make sure that the lf antenna is out of the nearfield of the hf antenna. if you cannot do this, note to expect some possible pattern deformation and impedance changes in the hf antenna.

good luck. should work well with careful design.

 

[davenn]

Hi Domu,

for 1GHz, I would recommend you DO NOT use any of those coax's you list, the losses in any of them over 50m is unacceptable. you should be using at bare minimum LDF 4-50 (1/2 inch), which will have 4dB loss over 50m at 1Ghz. And preferably LDF 5-50 (7/8 inch), which will have 2.2dB loss over 50m at 1Ghz. <<< very acceptable

I'm using the LMR/CNT400 type here on a 1296MHz 18metre run and have 3dB loss will be upgrading it to LDF4-50 in the near future

You never stated as to if this is for TX and RX... But I'm picking from you comments it may be RX only.
As stated by VFONE diplexers at each end is what your need to separate the freqs.

cheers
Dave
VK2TDN