[Milsurplus] LO radiation

[Bob Camp]

Hi

The three db isolation number is typical of a single diode mixer. I can 
dig out some papers if need be. I suspect it's also typical of the 
front end on a regenerative receiver. Better mixers or radios with RF 
stages would have significantly better isolation. Best guess would be 
in the 10 to 20 db range for single diode mixer designs with typical RF 
stages. A double balanced mixer gets you into the 30 db range with no 
RF amp and a matched antenna, less if there are mismatches in the 
system.

An antenna running at 0 dbi (decibels above isotropic) is a pretty poor 
antenna. A simple dipole running in a matched condition will give you > 
2 dbi. My assumption is that we were worried about the antenna running 
at it's best case (or worst depending on your view) condition. Even if 
the problem only exists over a 20 % region it's still a problem.

The mismatch issue is valid on the transmit end, but less so on the 
receive end. Depending on just how the antenna impedance interacts with 
the isolation number you could get all kinds of numbers on the transmit 
end. Receive antennas normally are not matched and act as voltage 
sources. I suspect that a reasonable DF set would try for as much 
antenna gain as possible without reducing the directivity of the setup.

One thing going in favor of the TX end of the setup. The antenna needs 
to be matched to the radio to some extend for it to receive properly. A 
high impedance antenna running into a low impedance receiver would be 
an issue for good reception. It's not a totally bilateral situation but 
at least it's in the right direction.

I did make the assumption that we are talking about shipboard 
installations. That limits the number and size of antennas that need to 
be considered. Five thousand foot long wires probably aren't anything 
we need to worry about. Assuming that the basic numbers are right and 
we are worried about frequencies below 10 MHz then the antennas would 
range from a couple of wave lengths down to a fraction of a wavelength. 
The longer antennas will have complex patterns of nulls and peaks, the 
shorter antennas will get down to what ever the matching gets them 
relative to 2 dbi. Mismatch probably kills you at some point in the MF 
region no matter how lucky you get. A couple of wavelength antenna 
could get you into the 6 db region if everything worked just right.

Again it's not a calculation of what *does* happen, it's only a 
calculation of what *could* happen. Without real data on real radios 
(and antennas - good point) you are never going to get real world data. 
If you were going to look at real data I think it you should look both 
at the LO and at harmonics of the LO. The combination of antenna gain 
and match might be better on the second or third harmonic ....

	Take Care!

		Bob Camp
		KB8TQ





On Mar 27, 2005, at 3:24 PM, Hue Miller wrote:

>
> ----- Original Message -----
> From: "Bob Camp" <ham at cq.nu>
>
>> Hi
>>
>> Ok, here's a stab at it. I'm assuming a single diode mixer. A common
>> setup in early IFF receivers. Simply to keep things straight the local
>> oscillator radiating radio is called the transmitter.
>>
>> TX LO power +3 dbm
>> TX isolation LO to antenna -3 db
>> TX antenna gain +0 dbi
>>
>> ERP from the TX 0 dbm
>
> TX isolation to antenna -3 db
> *This is RATHER optimistice, isn't it ?
>
> TX ant gain 0 dbi
> *Are you assuming a matched-antenna case? How was the receiver 
> "matched"
> to the antenna, and at what frequencies? If not, how does a severe 
> mismatch
> affect this figure, affect power radiated. I suggest that this figure 
> is VERY
> optimistic. And as we know from using long-wire antennas, the antenna 
> is not
> strictly reciprocal as long as it is longer than some (hi-reactance) 
> short length.
>
> The Navy list of "safe, approved" receivers had a list versus the 
> actual antenna
> ( note, antenna post or connection! ) power, didn't it? I can't find 
> mine right now...
> -Hue Miller
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