A solution to combining 2 antennas into 1 reciever,or 1 antennas into 2 recievers, as I have successfully done ,can be acheived like this. I will attach a photo, all points terminate as 50 ohms. You will need 50 ohm and 75 ohm cable and soldering iron.
The 2 bits of 75 ohm are a 1/4 of a wave length long but multiplied by .66
from the web 75 ohm coax cable segments have a V.P. of 66%. At this point, multiply the V.P. (.66)
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There are several problems here.
If youâre using this to split a single antenna between two receivers, the rtlsdr dongles are very noisy and feed noise back towards the antenna (& therefore the other dongle). You should use a proper power splitter/combiner with good isolation instead.
If youâre using this to combine two antennas, then youâre basically building an accidental phased array; thatâs not necessarily going to work well (youâll get directional effects)
Whatâs the rationale behind the mismatched impedances?
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No, just NO.
There is no universal velocity factor for any coax. It is a function of the materials used. As the materials used varies widely, so does the velocity factor.
Thatâs very polite - I would have said âa random phased arrayâ (the phase angle will change as the aircraft moves)
Playing with a wave tank in a High School Science Lab would quickly teach you that there are only two points where a plane could be where the signals would be in phase and therefore âaddâ to give a stronger coherent signal.
In all other positions there will be some degree of cancelling.
Instead of a square wave, the signal will start to look like a sine wave and greatly degrade decoding.
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Itâs to get a 50 Ω match at the common point.
On each of the 1/4 wave 75 Ω legs, the 50 Ω load will be transformed to 100 Ω.
These 2 100 Ω loads are then connected in parallel giving you a 50 Ω match.
So how do you match a 100 Ω feed to a 50 Ω antenna? (ignoring the need for 75 Ω connectors)
(this is largely a rhetorical question because (as above) I believe this approach is unsuitable for ADS-B reception)
Good point 
For matching impedance of Antenna with impedance of Feed Cable, the quarter-wavelength interposing matching cableâs impedance Zstb shoul be:
Zstb = â (Zfdr x Zant)
= â (100 x 50) = 71 Ω
Inserting a 1/4 wavelength 75 Ω cable piece between antenna and feed cable is the closest practical solution.
100 Ω feed coax >> 1/4 WL 75 Ω coax piece >> 50 Ω antenna
The problem that @geckoVN was alluding to is that when you look the other way, you have the same transformation.
So you have 50 Ω at the common point and this is transformed to 100 Ω at each of the antenna connection points to which you are connecting 50 Ω antennas.
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@LawrenceHill
As there is zero dB isolation between two antennas at T-junction, the system beyond 50Ω feed point acts as two 100 Ω impedances in parallel, i.e. a single 50 Ω load.
If a good quality satellite tv splitter is used, these normally have circuits which provide an isolation of 20~30 dB between out terminals.
Freq. DC-4200, Isolation 6.6db, Insertion loss .3db $59.95
Better Isolation, slightly more Insertion loss:
Freq. 950-1750, Isolation 30db, Insertion loss .5db $59.95
Freq. 10-1500, Isolation 30db, Insertion loss .5db $74.95
Good Freq range and Good Isolation
Freq. 10-2500, Isolation 17db, Insertion loss .6db $83.95
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This is the unit I personally use on my test rig:
You can usually find them on Ebay for ~$20-25 (or less depending on how patient you are.)
Must also bear in mind that using a power combiner/splitter is not free. Be sure to add +3db loss to whatever is published since splitting a signal also halves it.
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From the ZAPD-21+ datasheet linked:
Thatâs just BS.
Insertion Loss = Insertion Loss (not " Insertion Loss plus anything")
So the path loss for the ZAPD-21+ is 3dB + 0.25dB = 3.25dB (not 8 dB)
@abcd567 - quoting yourself doesnât make it âmore trueâ. If it was wrong the first time, it still is.
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I have said that 3dB should be added to the insertion loss declared by manufacturer/seller. You said the same thing, but reworded and rephrased it. If what I said is BS, then what you said is also BS.
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I did not âquoteâ myself to make it authentic, but only to save effort to re-write something I have already posted.
No - to both points.
You said " insertion loss = splittig loss + internal loss" - It isnât.
This is correct (for the path loss), but not what you said.
If you think my post was just a rewording of your post, that explains why you donât understand why you post is wrong.
Just to unstir the pot a bit:
He was talking about really bad splitters.
But an insertion loss of 5dB would just be astronomical.
Iâm not sure internal loss is a term that is really specified in that context, is it?
Best to just avoid it if it is.
I use https://www.minicircuits.com/pdfs/ZESC-2-11+.pdf as a splitter for two SDRs.
It has the advantage, for my aplication, of having SMA conectors.
You are right that what @geckoVN quoted from my post was about insertion loss in common commercial TV/Satellite splitters, and NOT professional splitters like minicircuits.
Below is photo of a commercial tv/satellite splitter and if you look to its label it says:
Low loss output
5 MHz - 1 GHz: -3.5 dB
1 GHz - 2 GHz: -4.9 dB
Here is another TV/Satellite splitter
Insertion loss 4.4 to 6.4 dB over frequency range (5 MHz to 2.4 GHz)
âPath lossâ is the term coined by you, not a standard technical term used for splitters. Path loss may describe transmission line losses, but in splitters 3dB loss is not due to path, it is due to bifurcation or splitting of incoming signal into two equal halves.
