Joe M (Voyager), based on your response to my suggestion of using a diplexer type of device for a multi antenna configuration, it appears you may not have a very comprehensive background and understanding of RF energy. The specific area of concern when combining multiple antennas has to do with multipath or phase interference/cancellation.
The suggestion to use a multicoupler/splitter for the 2 drastically different antennas is most likely prone to unusual and unexpected degradation in performance, not to mention the insertion loss of the device. Multiple antennas can be combined, however, these are usually configured under specific conditions to create co-phased or multi-phase antenna arrays.
It is not clear what bands the original poster was interested in, however, I am assuming the Yagi “band” would be connected to the appropriate band pass input port, while the “all band” antenna would be connected to the other input port. I was also assuming a 2-port device with the appropriate frequency bands for this application exists. This configuration would not attenuate the Yagi performance other than the diplexers minimal insertion loss. The “all band” antenna would then cover the other portion of the band that the diplexer did not block out, again with minimal insertion loss.
There is some concern about using an “all band” antenna and a diplexer. A diplexer is really a filter that will limit the frequency band any antenna feeds into the common port of the diplexer, so this does not eliminate the use of an “all band” antenna. It really does not matter if a wideband or frequency specific antenna is used with a diplexer. Ideally, an antenna that is optimized for the specific band and application should be considered.
It is debatable if a true “all band antenna” actually exists. Most antennas exhibits some form of “wideband” performance, however, the efficiency and radiation pattern beyond the design limits might vary a great deal. Some antennas, like the Austin Ferret, are specifically designed to be optimized over different band segments, however, it performance will vary with frequency.
The so-called “all band” antennas for around $50 are usually a basic compromise in performance. They do work over a wide frequency range; however, they may not be the best solution for the application. In certain situations, you may find that a well-tuned 800 MHz antenna covers the VHF and UHF bands quite well for your area. This would more likely be due to the high signal level of the VHF and UHF systems and less likely due to the wideband performance of the 800 MHz antenna.
Depending on the type of monitoring a person does, there are different antenna configurations that might be suggested. I split radio listening into the follow two groups:
1.“Signal Monitoring, Searching and Hunting”: usually an operator interactively and decisively looks for signals in a specific band segment. For this style of monitoring a coaxial switch or manually connecting different antenna feeds is probably acceptable.
2. “Scanning”: usually an operator has the radio following a preset group of frequencies, that may cover many bands, and the radio is operating without direct user involvement. This usually requires a single coax feed with the signals that the operator is attempting to capture. Use of a diplexer would allow multiple antennas for specific bands to be combined without much signal loss or concern about multipath phase interference/cancellation.
JamesO
JamesO said:
Joe M (Voyager), based on your response to my suggestion of using a diplexer type of device for a multi antenna configuration, it appears you may not have a very comprehensive background and understanding of RF energy. The specific area of concern when combining multiple antennas has to do with multipath or phase interference/cancellation.
Diplexers only work on two frequency bands - and even then in a very narrow range. They will not work for one wideband antenna and one narrowband one. It appears I'm not the one lacking backround in RF energy.
I've made many (hundreds) of diplexers for use with low band wide band mobiles (usually 33 and 46 MHz). I know what they do and how VERY well.
The suggestion to use a multicoupler/splitter for the 2 drastically different antennas is most likely prone to unusual and unexpected degradation in performance, not to mention the insertion loss of the device. Multiple antennas can be combined, however, these are usually configured under specific conditions to create co-phased or multi-phase antenna arrays.
The insertion loss will be 3dB. I SAID that before. Yes, with two drastically different antennas, the results could be unpredictable, but not enough to make it useless. You know the loss on each (3 dB). You know the gain of the yagi and the wide band antenna. The only tthing you don't know is how far apart the antennas are. This will be the only factor determining phase cancellation, and will be in different directions depending on the RX frequency.
It is not clear what bands the original poster was interested in, however, I am assuming the Yagi “band” would be connected to the appropriate band pass input port, while the “all band” antenna would be connected to the other input port. I was also assuming a 2-port device with the appropriate frequency bands for this application exists. This configuration would not attenuate the Yagi performance other than the diplexers minimal insertion loss. The “all band” antenna would then cover the other portion of the band that the diplexer did not block out, again with minimal insertion loss.
The 'band pass input port'? Well, a simple splitter has no such port, nor does a diplexer. It sounds like you're talking about a DUPLEXER - MUCH different than a DIPLEXER.
NOBODY - REPEAT - NOBODY makes a diplexer that covers a wide frequency range. It's just not possible due to the way a diplexer works. Basically, a diplexer nulls out a narrow frequency range so the antenna on one leg is non-existant to RF on the opposite frequency. And it's not possible on a duplexer to make one that is wideband on one port, and frequency range specific on the other. If you do, it's a simple splitter (like the one I posted a link for) with a bandpass cavity on one leg.
There is some concern about using an “all band” antenna and a diplexer. A diplexer is really a filter that will limit the frequency band any antenna feeds into the common port of the diplexer, so this does not eliminate the use of an “all band” antenna. It really does not matter if a wideband or frequency specific antenna is used with a diplexer. Ideally, an antenna that is optimized for the specific band and application should be considered.
Again, it sounds like you are talking about a DUPLEXER, not a DIplexer. A duplexer, for the most part, is a high pass - low pass filter that separates two (or more: 3=TRIplexer, Etc) frequency ranges - typically with a crossover of 500 kHz to 3 MHz. A DIplexer, on the other hand, is a device that, usually using a specific length of coax cut based on the tuned frequency of the opposite antenna, notches out the frequency range of the 'opposite' antenna. This can also be done using notch cavities, but in either case, the notch must be tuned to a single frequency or narrow frequency range.
It is debatable if a true “all band antenna” actually exists. Most antennas exhibits some form of “wideband” performance, however, the efficiency and radiation pattern beyond the design limits might vary a great deal. Some antennas, like the Austin Ferret, are specifically designed to be optimized over different band segments, however, it performance will vary with frequency.
It is a fact that any antenna designed for a more narrow frequency range will have more efficiency on that range as opposed to an antenna designed to cover many frequency ranges.
2. “Scanning”: usually an operator has the radio following a preset group of frequencies, that may cover many bands, and the radio is operating without direct user involvement. This usually requires a single coax feed with the signals that the operator is attempting to capture. Use of a diplexer would allow multiple antennas for specific bands to be combined without much signal loss or concern about multipath phase interference/cancellation.
JamesO
A diplexer will work on TWO antennas, and ONLY two. Since the feedline on the 'opposite' antenna is cut based on the frequency of the other, it is not possible to cut it to two frequencies. Same with a notch filter - it's limited to notching ONE frequency.
I'll give you another example. Give me the dimension (singular) of a quarter wave antenna that will operate correctly on 150 and 300 MHz. If you can do that without changing the laws of physics, I'll accept everything you say as gospel.
No matter what - when you conbine two antennas, you will lose 3dB on each. The ONLY way to avoid this is by using a duplexer (or triplexer, or quadraplexer, Etc) that makes EACH antenna invisible to the others through the use of bandpass filters, such as those made by Comet or Diamond. However, note that these are NOT diplexer units. They merely separate frequency bands.
Joe M.