Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KB9MZ, Mar 8, 2011.

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1. ### KB9MZHam MemberQRZ Page

What are the things that make an antenna a "receive" form "only" when receive and transmit are supposed to be reciprocal when a direct 1;1 SWR exists without the use of a matching system?

2. ### VK1ODHam MemberQRZ Page

Perhaps you need to give some examples.

The reciprocity theorem for an 'antenna' does not imply reciprocity for the entire system. Doubling antenna gain (ie a 3dB increase) does double EIRP in a linear system, but for instance, it does not necessarily improve local received S/N ratio by 3dB.

Owen

3. ### KB9MZHam MemberQRZ Page

An example would be a meander form of high density within a small circle. This will have a low noise factor if shielded
because of low local noise pick up, but it can be resonant even tho having a small apartuure. However, because of the meander
construction it can easily be resonant by not being electrically small. Another example is a multi turn loop antenna where the
turns are reversed with respect to each other

4. ### KL7AJHam MemberQRZ Page

Although transmit and receive gain and radiation pattern are reciprocal, the REQUIREMENTS aren't A very small loop, for example, can make an excellent low noise antenna, but the radiation resistance is extremely low, and does not make an efficient transmitting antenna.

Sometimes having a sharp null in the pattern is very useful for a receiver antenna...either for direction finding or noise reduction, but it is seldom a good thing for a transmitting antenna.

Certain antennas like the Beverage are very good receiving antennas, but are totally unsuitable tor transmitting.

Hope this helps.

Eric

5. ### W0BTUHam MemberQRZ Page

Well said, Eric.

I spent most of the evening working on and listening on my Beverage receiving antennas. I'm slowly putting together a web page about them at http://www.w0btu.com/Beverage_antennas.html.

6. ### W5DXPHam MemberQRZ Page

The primary factor is the dynamic range of the receiver, e.g. being able to amplify the received by 70 dB or more, an advantage not shared by a transmitter. So as long as the signal to noise ratio doesn't decrease during receive, the efficiency of the receiving antenna doesn't much matter up to a point. A 0.1% efficient receive antenna usually works just fine for receiving.

7. ### KB9MZHam MemberQRZ Page

I appreciate the responses but are they not ignoring the fact that the antenna is resonant?
A case in point. A foot cube box of wire is crambed full with wire that happened to be resonant with a 50 ohm impedance.
If it was receiving S9 signals on top band what could happen to prevent reciprocity in transmit assuming that the radio is working correctly?

8. ### KA4DPOPlatinum SubscriberPlatinum SubscriberQRZ Page

If you had a box containing an electrically resonant length of wire and it showed 50 ohms at the feed point at it's resonant frequency then you should call the Pope because a miracle just happened. As far as reciprocity is concerned you wouldn't need to worry about that since the signal on the wire running in all directions inside the box would mostly cancel so I guess you would have a cheesy dummy load.

There are a whole bunch of antenna books that can be very helpful in situations like these, I would be happy to recommend some if you like.

9. ### KB9MZHam MemberQRZ Page

To cut to the chase. It has been stated that you can cram a bunch of wire into a small space where the wire
is at least a wavelength and is resonant at 50 ohm impedance. But the "fact" remains that it is still a short antenna!
If this is really a "fact", and it is often stated on this newsgroup, what happened to the supposed reciprocity theorem? WHAT made it a " receive" antenna only
The question comes about because of the fact I actually posses such a compressed antenna array for top band and I want to know what are the actual circumstances that could make it a "receive" antenna only?

10. ### N1CZZXML SubscriberQRZ Page

This thread is good timing for me. I been thinking the last few days that I need to research received antennas a little more. Already here in Florida I am getting very high static/static crashes, as if it was summer. I have an antenna that works great on 160-40m, but with 10-20 over static crashes, it doesn't do me too much good.