System for Distribution of a Receive-Only Antenna to Multiple Receivers
I have twenty seven (as of this writing) receivers, including the receive function of some transceivers. I like to use a few - or all - of them at the same time. I generally want the same antenna on all the receivers. So it is necessary to distribute the receive antenna signal to all of the receivers.
The simplest way to do this, which I have done in the past, is to simply wire all the receivers to the same antenna, as illustrated by the schematic below in Figure 1.
Where the receiver is really a transceiver, I connect to the Receive Input antenna terminal so that an inadvertent transmission can't fry all the receivers. This is obviously a suboptimal arrangement: the impedances are badly mangled and the sensitive front ends of the receivers are all directly interconnected. In a process that I do not fully understand, I have blown up the front ends of several modern transceivers when they were wired this way along with some vacuum tube boatanchor receivers.
I think there are several potential sources of the problem. Perhaps some voltage in some boatanchor is not adequately DC blocked under some circumstance - maybe while switching - allowing this DC voltage to be momentarily applied to all of the receivers, thereby, frying the front ends of some or all of the solid state rigs. Or perhaps it could be a burst of Local Oscillator energy that gets on the antenna terminal of some boatanchor under some certain condition. It could also have to do with static electricity. Although it doesn't actually fit all the symptoms, such as when the condition occurs, I suspect the problem is that too much transmit RF is picked up by the Receive-Only antenna and all the receivers, despite the fact that I switch it to ground upon transmit. Maybe the grounding relay contact bounce is enough to allow some transmit RF to get through. Whatever, I have multiple repair bills for various solid state gear that attest to the reality of the problem. Also, somewhat surprisingly and interestingly, the Racal RA6790, the RA6790/GM, and the Icom PCR2500 have been the most sensitive to this problem.
A much better solution is to use actual power splitters to distribute the signal. The splitter presents a 50 ohm load to the antenna and provides a 50 ohm impedance to each of its several outputs. I chose to use the Advanced Receiver Research PD4AB power splitter. This unit provides 20db of isolation between output terminals, which successfully stopped the carnage of the front ends of my solid state rigs. Here, in Figure 2, is the schematic of the basic setup - using only one splitter.
The problem with this, of course, is that we have served up the antenna signal to only four receivers. But not only that, the signal to each receiver has been reduced by about 7 db. So to fix these two little problems, we add another layer of power splitting, as seen in the schematic below, aka Figure 3.
We're getting closer, but what about the other eleven receivers that haven't gotten the antenna signal yet? Not only that, the signal delivered to each receiver is now down about 14 db. Not so good.
So let's add some more splitters and a preamp. Let's also cut a corner. This will get us more outputs and regain the signal strength that has been lost by all the splitting. This leaves us with the circuit shown below in Figure 4.
What I have done here is to wire the inputs of two of the splitters together, repeating - at least partially - the sin of the original circuit. However, the splitters don't care too much - they aren't going to zap each other - and the impedance mismatch is not nearly as bad as it was with all the receivers connected directly to the antenna. None of the receivers commit suicide now, which is good. The signal should be attenuated by something like 3 - 6 db when connected this way, making the overall system gain down by 17 - 20 db plus coax loss. (I should also terminate the one unused output with a 50 ohm load, but I haven't bothered yet). So this system serves all the receivers - but the signal is way down.
The gain issue can be dealt with by adding a preamp before all the splitters. Placing the preamp before the losses introduced by the splitters preserves the system's noise figure. I chose the ARR P0.5-30/20VD preamp. This is a broadband preamp that covers from .5 Mhz to 30 Mhz. Its gain is 20 db, which serendipitously just about exactly cancels out the 20+ db loss of the splitters. So now the gain of the system is just about neutral with respect to the antenna being directly connected to one receiver. The system now looks like Figure 5 below.
The measured signal is now about 2 db higher than the signal when it is run directly to one receiver, which is a little better than expected. This could be because the preamp's gain is more than the spec sheet 20 db, which is no doubt an average or minimum value (I haven't bothered to measure it). It could also be that the signal is not being attenuated as much as expected by the direct wiring to the inputs of the first two splitters. This could be because the preamp can drive the two splitters better than the antenna itself can. Said differently, the preamp output is not loaded down as much by the splitters as was the antenna itself. Which makes sense to me because the amp is an active device and can supply plenty of current - even into a bit lower impedance.
Here are a couple of photos of the system. Splitters are scattered around the station, generally being located in the vicinity of the receivers that they service. The smaller unit in the first photo is the broadband preamp from Advanced Receiver Research. The larger unit, with one input and four outputs, is one of the splitters.
The system works like a charm and I'm very happy with it. The performance of the preamp seems to compare quite favorably with my 756PRO. But I haven't measured such things as IMDR, so I can't say much that is quantitative. However, the third order intercept (TOI) of the ARR amp, as measured by W8JI, is better than the claimed TOI of the 756PRO. Qualitatively, it is quite good - surprisingly so actually.
I've just acquired a DX Engineering RPA-1 preamp, which has a considerably better third order intercept point (43 db) than the ARR amp (30 db) - based on tests done by W8JI. Haven't tried it yet. I plan to put both preamps in a relay switched enclosure so I can A/B them as I'm operating.
I also admit that I'm $934 plus shipping and coax and connectors into this "receive antenna distribution" project. So I realize not everyone will want to rush out and duplicate it. I know I could have saved a bundle by building my own splitters, and maybe I'll do that someday. But in the meantime, I'm up and operating and smiling. Otherwise I would still be winding toroids in my sleep. I rationalize the cost as a pittance compared to the cost of the 27 receivers themselves.
Whatever. I like it, it works well, and it makes me smile when I use it. Which, I think, is what Ham Radio (or any hobby) is all about..
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