Signal combiner for two Pi's one antenna


#1

I was looking at feeding my two Pi’s from one single antenna. Any recommendations for a splitter/combiner that would be decent on the cost but work great.

Thanks,
Glenn


#2

Couple of years ago, I purchased a bunch of these, but never used.
Don’t know how it will perform.

US $4.79 for 10 pieces
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US $1.99 for 5 pieces


#3

Due to splitting of signal into two parts, each receiver gets 1/2 the power, i.e. “splitting loss” = 3 dB for an ideal lossless splitter. Practically there is an additional internal loss of about 2 to 5 dB depending on quality of the splitter.

Hence:
insertion loss = splittig loss + internal loss = 3 dB + 2 to 5 dB = 5 dB to 8 dB
NOTE: Many manufacturer/seller specify "internal loss" as "insertion loss", omitting 3 dB "splitting loss" to give a false impression of a very low loss splitter
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Here is one example (NOT used by me, shown here for guidance only)

Specifications

  • Frequency: 2.4 GHz
  • Insertion Loss:
    5-1000 MHz: ≤ 5.5 dB;
    1000-1750 Mhz: ≤ 6 dB;
    1750-2450 Mhz: ≤ 7 dB

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#4

I used one of these for a while (bought brand new at a swap meet for $AU5)

It worked very well and the insertion loss was the same on both ports.

It was/is great for comparing amplifiers and attenuators by keeping one system as a reference and changing one thing at a time on the other. It is also good for evaluating dongles and gain settings.

S.


#5

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How many dB insertion loss, any figure specified by the manufacturer?


#6

I can’t find the specs for that particular model but here is a similar one with slightly different band width.

Operating Frequency Band 555 – 2700 MHz
3rd Order IMD -130 dBc
3rd Order IMD Test Method Two +43 dBm carriers
Average Power, maximum 80 W
Impedance 50 ohm
Insertion Loss at Frequency Band 0.3 dB @ 555–960 MHz | 0.4 dB @ 1710–2700 MHz
Isolation at Frequency Band 17 dB @ 555–698 MHz | 20 dB @ 698–2700 MHz
Peak Power, maximum 1 kW
Power Rating, combining 0.5 W
Power Rating, Splitting 80 W
Reflected Power, maximum 3 W
Split Loss 3.0 dB
VSWR 1.2:1 @ 698–2700 MHz | 1.43:1 @ 555–698 MHz

Full specs can be found here.

S.


#7

It is a good one as far as loss is concerned:

Total loss = Split loss 3 dB + Insertion loss 0.3 dB = 3.3 dB


#8

Need LNA instaled before a splitter.


#9

Not necessarily.

The loss due to the splitter I used is under 4 dB.

There is no end of posts about the need to reduce the receiver gain by 10 or 20 dB so there is often an abundance of signal. (Yes, an abundance is a genuine measure of excess signal).

The way I used it was to set up two identical receiver system using the same signal from the antenna. I then left one exactly as it was and did all my experiments on the second.

This gave me a direct, real time method of measuring the effectiveness of each change to hardware, software or parameters such as receiver gain.

In this environment it was about comparing each element and then using all that information to build an optimised system to suit my needs.

S.


#10

These super-cheap ones are just dumb splitters with no isolation between the output ports.
They will not work well with a rtlsdr, as the rtlsdrs feed quite a bit of noise (internal oscillator signals, etc) back towards the antenna connection, and without a proper splitter with good isolation, they’ll interfere with each other.

You can usually find proper splitters for a reasonable price second-hand (I have a mini-circuits ZESC-2-11 which works fine, nominally 19dB isolation / 0.5dB insertion loss). But maybe not for $5 :slight_smile:


#11

@obj
You are right. Fully agree.

The supper-cheap ones are solid T connection, 0 dB isolation.

The regular one from Amazon has high insertion loss of 3 dB + 3dB slitting loss = 6 dB loss, but has 17 dB isolation.


#12

The specs of splitters in market are confusing / misguiding / incomplete

The insertion loss is specified in three different ways.
For example for exactly the same insertion loss (say 0.5 dB), different seller/manufacturer specify as follows:

  • Manufacturer #1: Insertion loss 0.5 dB
  • Manufacturer #2: Insertion loss 3.5 dB
  • Manufacturer #3: Insertion loss: 0.5 dB, Splitting loss = 3 dB

Also

  • Some will specify frequency band only, nothing about insertion loss or isolation.
  • Some will specify frequency band and isolation, nothing about insertion loss.
  • Some will specify frequency band and insertion loss, nothing about isolation.
  • Some will specify all: frequency band, insertion loss, and isolation.

No 2-way splitter can have less than 3 dB TOTAL loss due to splitting of signal power into two halves (1/2 = - 3 dB)


#13

They show up on eBay from time to time. Never for $5, but $20-25. There is one right now for $22.50, others at a higher price.


#14

I was wondering why I purchased these super-cheap solid T splitters, and then why never used these.

Now I recall that I did not purchase these to split single antenna output to two receivers.

I have purchased these to combine two yagi/corner antennas to feed one receiver by making a phasing harness.

Since I never made a yagi or a corner antenna, these Tees were left unused.


#15

Same here. A CATV splitter is still the most affordable/practical option for one antenna-two receivers, losses and all.


#16

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The above two statements support each other … particularly if you are using ProStick with integral 16 dB rf amplifier.


#17

And some will add an additional amp to it. More than enough gain to handle any loss.


#18
Type Insertion loss Splitting loss Total loss
Good quality, high cost $25 to $70 0.2 dB 3 dB 3.2 dB
Medium quality, moderate cost $5 to $10 6 dB 3 dB 9 dB
Low quality, low cost $2 to $3 9 dB 3 dB 12 dB

Going from high quality to medium quality means an additional loss of 6 dB (recoverable by ProStick’s rf amp), but gives a substantial price saving.


#19

Make sure the frequency range of the splitter covers the frequency of the ADS-B signal; 1090 MhHz!

I also found that the cheaper splitters were not quite even in the split with slightly more power going to one port compared to the other.

Not a problem if you know about it and compensate appropriately.

S.


#20

I’m very satisfied with the ZAPD-21 splitter; specs ==> https://ww2.minicircuits.com/pdfs/ZAPD-21.pdf

My small 5/8 antenna feeds my RasPi 3 and Raspi Zero.