Importance of a 'tuned' receive antenna?

[KD0TAZ]

Ok, so I know about the importance of having a properly tuned antenna for transmitting. If it's not tuned, your range (and probably finals) will suffer. That's ham 101. But I never understood the need or recommendations from people to have 'band appropriate' antennas for a receive only application like scanning. When it comes to receiving, I always thought there were only 3 rules: get it outside, get it high, and bigger is better. For example, I am using a retired half wave copper cactus up on my roof for my scanner. It is tuned for 146MHz (~38"). But I can pick up the 800MHz system from the next big town 30 miles away. It's not pulling it in 100% (due to the terrain and I assume limited transmit power), but I honestly can't see how chopping 30+" off the top of it to 'tune' it for 800MHz would make it any more effective at receiving the weak signal. I'd think it would just make it worse. Am I flawed in my thinking here? I mean, I can understand if you needed a directional for whatever reason (namely TV) using something band specific, but not for general scanning.

I don't know, maybe I'm missing something here?

Thanks

 

[N1BHH]

Band specific is not what I suggest. A purpose specific antenna is the best way to go, but anyone can use a VHF quarter wave whip and receive quite well. Any antenna will work on your scanner. Heck, I hooked up my Wilson 1000 to my scanner a few times and didn't notice anything like poor reception of UHF or 800 Megahetz signals.

I encourage people to try something that is different, even simple. A wire hanging out the back of a scanner will work, it won't be hearing the world, but it works. The old adage of Keep It Simple Stupid fits into scanning as it does everywhere else.

 

[n5ims]

A tuned antenna for a receive only application is very important if that's the only way to pull in the desired signal, but in the rest (probably 95%+) of the cases it really doesn't matter. As long as whatever you have hanging out there pulls in enough signal to do the job, you're good!

 

[K9WG]

The only time you would notice a difference is if you are trying to pull in a very weak signal. Now if you are trying VHF/UHF DX you might want to go with a tuned antenna and low loss coax. Otherwise most cases it doesn't matter.

BTW as far as transmitters, you don't need a perfectly tuned antenna. It is desirable to have one but not required. Most modern radio transmitters have protection built in and will reduce power in relationship with the mismatch.

 

[n8zcc]

BTW as far as transmitters, you don't need a perfectly tuned antenna. It is desirable to have one but not required. Most modern radio transmitters have protection built in and will reduce power in relationship with the mismatch.

It should be noted that the more dis-tuned the antenna is to the transmit frequency, the more transmit power that is not radiated and is returned to the transmit and dissipated as heat.

 

[Fred1]

Receiving Antennas

I noticed a gain at least of 2db using a 1/4 wave 800 mhz up 40 ft with rg-6 vs an all band scanner antenna,
Antenna was wade from 10 copper wire and a SO 239 center freq is 855.000. I can hear all counties around me better and 2 state systems also.

 

[W6KRU]

In my experience a receive only antenna for a lower frequency will work fine at the higher frequencies but not the other way around. An antenna that is too short will not work well but if it is too long,it will be OK. You can receive 800 on good VHF-Low band antenna but not the other way around.

 

[davidgcet]

a resonant(tuned) antenna will give maximum signal out for TX and maximum signal in for RX. RX is not AS critical, but there will be some improvement in tuning when done properly. any antenna rule that applies to TX applies to rx(gain/loss/SWR/impedance matching), it is the SAME thing just a much lower power level.

 

[prcguy]

It depends largely on the type of antenna and feedline used. For example, on HF a random length dipole fed with 300 or 450ohm balanced line will not benefit noticeably from trimming to the exact length for a specific frequency except for the lobes or nulls created when the antenna is longer than 1/2 wavelength.

For a shortened loaded HF antenna fed with coax performance drops off drastically when the antenna is out of resonance.

For VHF/UHF scanner antennas (other than Discones) it can be similar with the antenna performance peaking at a few discreet frequencies where it resonates and then dropping off to nothing out of band.

A good example of this would be in a recent Monitoring Times magazine article where the author compared a wide band 30-512MHz military vehicular antenna to a few common mobile scanner antennas and you can see the dramatic peaks and loss in performance on the scanner antennas.
prcguy

 

[nanZor]

From a systems standpoint, if your copper cactus is allowing you to monitor all that you need to hear, then there is no need for higher performance.

One thing to remember is that when you lengthen an antenna beyond a quarter-wave vertical, or a half-wave dipole, the directional characteristics change, among other things. This may or may not be a problem depending on your needs, especially for rx-only.

Example: Your copper cactus half-wave has a directional pattern at VHF that looks out mostly horizontally towards the horizon, and not much sensitivity to signals arriving from straight above. Yet I'll bet that you can even hear UHF comms 446 mhz ok too. However, if you were able to actually "see" the directional pattern at 446 mhz in front of you with your 2-meter cactus, you'd see a "cloverleaf" pattern more or less so the antenna will respond best to UHF signals arriving from about 45 degrees above the horizon. This might be ok for UHF repeaters if you are in a valley, but the mobiles will be weaker than had you used a half-wave cactus cut specifically for uhf.

Some are actually able to accept this higher look angle on uhf and put their standard 2-meter j-pole into dual-band use. Not ideal, but it works for some.

Now on 800mhz, even if you have a good match on with your existing cactus, and low loss coax, your antenna is looking for the strongest reception practically straight up and not at the horizon. So the 800mhz systems you are hearing are just really strong. If you had an antenna cut specifically for 800mhz, say a simple 6-inch vertical dipole, the strongest reception would be at the horizon again, and not straight up, like your cactus is doing now.

This is similar to the popular "OCFD" vertical homebrew wideband dipole here which is about 4-1/2 feet long. Yes, it matches well across a wide spectrum, BUT as you go higher in frequency beyond VHF, the antenna is looking high in the sky for best reception. That compromise is ok for some, but most would not claim it to be "high performance" on it's own merits without taking into account the "system" that works for you.

One thing that might be helping you, is that at 800 mhz, the common-mode of the coax will be skewing the directional pattern of your 2-meter cactus for a lower look angle! I haven't seen any common-mode chokes that work at 800mhz, so your transmission line might actually be assisting in your 800 mhz reception by unintentionally skewing the pattern more favorably. Usually this isn't desired. The "OCFD" suffers from this same issue, but if the unintenional skewing helps, then it's all good.

To sum up - when the antenna is longer than normal, the best reception is usually no longer broadside to the element, but starts to move more towards the ends. Whether this is an aid or a hindrance is part of the design decision.

 

[kb2vxa]

OK, to straighten this mess out, that copper cactus may not be band specific for 800MHz but it IS resonant 7/4 waves worth so it exhibits a bit of collinear gain. It's pretty close to 50 ohms too so it's a good impedance match to the normally used type of coax.

 

[prcguy]

An antenna should only be considered a "collinear" if elements (usually half wave) are arranged in phase so maximum gain is achieved in a desired pattern and usually at the horizon.

An antenna that is simply 7/4 wavelength long would have gain but not at the horizon and not as much if separate elements were assembled at optimum spacing.
prcguy

OK, to straighten this mess out, that copper cactus may not be band specific for 800MHz but it IS resonant 7/4 waves worth so it exhibits a bit of collinear gain. It's pretty close to 50 ohms too so it's a good impedance match to the normally used type of coax.

 

[jackj]

The same physics that describe why and how an antenna works on transmit also apply on receive. Remember that the only difference between a receive and transmit antenna is the direction the signal flows. A non-resonate antenna will work on transmit and receive but don't expect it to work as well as a resonate antenna.

The impedance of a non-resonate antenna could be anywhere from 1 or 2 ohms up to several thousand ohms. This is a very large barrier to efficient energy transfer. Another thing to consider is that an antenna absorbs the energy of the magnetic field most efficiently when it is a multiple of 1/4 of length of the wave.

None of this means that you can't use multi-band antennas. Just don't expect a discone that covers from 27 Mhz to 1500 Mhz to be as efficient as a resonate 1/4 wave antenna at 54 Mhz..

 

[prcguy]

A Discone designed for say 25 to 200MHz would be within about 1dB of a tuned resonant 54MHz 1/4 wave ground plane at the horizon.
prcguy

Snip...
None of this means that you can't use multi-band antennas. Just don't expect a discone that covers from 27 Mhz to 1500 Mhz to be as efficient as a resonate 1/4 wave antenna at 54 Mhz..

 

[WA1ATA]

A Discone designed for say 25 to 200MHz would be within about 1dB of a tuned resonant 54MHz 1/4 wave ground plane at the horizon.
prcguy

Is that per the manufacturer's claims, independent measurements, or just a supposition on your part?

 

[prcguy]

Based on various military and commercial specs and antenna design books and some measurements.
prcguy

Is that per the manufacturer's claims, independent measurements, or just a supposition on your part?

 

[WA1ATA]

Based on various military and commercial specs and antenna design books and some measurements.
prcguy

Thanks.

I don't have the instruments to compare my RS 20-176 wideband scanner antenna with tuned dipole variant antennas, but I would have guessed a bigger variation between that antenna and tuned antennas, just based upon the effect on the reception of weak signals. Of course, the 20-176 is just a 20" (approx) 1/4 whip with 3 drooping ground plane radials and a couple of smaller parasitic verticals.

If the difference between a tuned antenna and the discone is only 1 dB, I'll have to either build and buy one.

 

[prcguy]

A dipole has slightly more gain than the 1/4 wave ground plane I compared to a Discone and there are subtle differences between Discones, so its hard to put an exact number on how much better or worse you might receive with a ground plane, Discone or dipole cut for a specific frequency.

I will say for consistent gain or performance across a wide range of frequencies, its really hard to beat a Discone antenna. Their rated for about an 8:1 frequency range but toss out the upper 1/3rd or so due to the pattern shifting upward if you want more predictable performance. Even with that reduced frequency range there is nothing affordable that I can think of for scanner use that will equal the bandwidth vs gain of a Discone. You want more gain then the antenna will only cover a few spot frequencies due to the nature of phasing resonant elements for gain.

There are some exotic military and EMI test antennas that offer a little more gain than a Discone with almost as much BW but their not cheap. Some newer military antennas that cover continuous from 30 to 512MHz have technology that is not secret or new but for some reason it hasn't been used in the scanner industry.

One method of broad banding the 30-512 MHz military antennas seems to be breaking up an element with the equivalent of traps at strategic points. It has some losses but overall these antennas appear to work fairly consistent across the entire range.

I think the December issue of MT magazine has the article where a broad band military antenna was tested against some scanner antennas and the resulting chart is what this thread is all about. You can see the resonant points of the scanner antennas in the 150 and 450MHz range and a peak within the VHF lo-band, and everywhere else the gain falls off the map compared to the broad band mil antenna.

So if you want to use your tuned for commercial band scanner antenna in the 225-400MHz UHF air band it might be down by 25dB. Or worse yet the scanner antennas tested in the article also had loading coils for VHF lo-band and you can see the sharp peak where the antenna is resonant then it might drop 40dB within the same band where its not resonant. Get the article, I think its interesting reading for antenna aficionados.
prcguy

Thanks.

I don't have the instruments to compare my RS 20-176 wideband scanner antenna with tuned dipole variant antennas, but I would have guessed a bigger variation between that antenna and tuned antennas, just based upon the effect on the reception of weak signals. Of course, the 20-176 is just a 20" (approx) 1/4 whip with 3 drooping ground plane radials and a couple of smaller parasitic verticals.

If the difference between a tuned antenna and the discone is only 1 dB, I'll have to either build and buy one.

 

[kb2vxa]

Typical RR technogeeks spot one small inaccuracy and rip a post to shreds, then continue on technobabbling to confuse the poor guy who asked a very simple question and out of all got ONE simple answer. If nothing else you provide fine entertainment but little useful information so keep up the good work, CONFUSION RULES! (;->)

In days of old when writers were bold
Before internet was invented,
I put my quotes in quotation marks
Just to piss off the snarks
And walk away contented.

<rrrraaazzz>

 

[K9WG]

Typical RR technogeeks spot one small inaccuracy and rip a post to shreds, then continue on technobabbling to confuse the poor guy who asked a very simple question and out of all got ONE simple answer. If nothing else you provide fine entertainment but little useful information so keep up the good work, CONFUSION RULES! (;->)

In days of old when writers were bold
Before internet was invented,
I put my quotes in quotation marks
Just to piss off the snarks
And walk away contented.

<rrrraaazzz>

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