or Connect
AVS › AVS Forum › HDTV › HDTV Technical › How to build a UHF antenna...
New Posts  All Forums:Forum Nav:

How to build a UHF antenna... - Page 22

post #631 of 4787
Quote:
Originally Posted by 300ohm View Post

In that scenario, the input to the vhf portion of the preamp from the uhf antenna is liable to be much greater than 3db because of the L/C (coil and capacitor) filters for each band. Ideally designed, the L/C filter should cut ALL the signal from outside of its frequency range 100%. But in real life its never 100% so some UHF signal will get thru, as will all the VHF signal on that VHF preamp input.

Modifying the preamp is a possibility, but without having high frequency RF test equipment, chances of success are close to nil.

The 3db loss I mentioned does come from the combiner/antenna in a different direction, which youll have to use on the one uhf preamp input, in that scenario.

So to clarify

Scenario 1:
Single compromise antenna receiving RF9 through RF48 connected to a single input preamp designed to pass both VHF and UHF such as CM0064.

Scenario 2:
Antenna 1 targeted at RF9 and attached to VHF input of 2 input preamp such as CM0264.
Antenna 2 targeted at RF18 to RF48 attached to UHF input of the same preamp as Antenna 1.

So Scenario 1 would actually have less signal loss because it is not filtering anything and then recombining? That does seem logical.

How do the 2 scenarios affect the Signal to Noise ratio? Would the S/N for both be the same, just a lower output in Scenario 2, or do components other than the first transistor add more noise?

Thanks
post #632 of 4787
Quote:
Originally Posted by rmcneil321 View Post

Could you clarify the 3db loss? I'm not talking about using a combiner for 2 separate antennas. I'm considering 2 antennas on a pre-amp with 2 inputs, one VHF and one UHF. Isn't this essentially the same thing as having two antennas on two pre-amps that are then combined but the pre-amps prevent backward losses from the opposing antenna?

Quote:
Originally Posted by 300ohm View Post

In that scenario, the input to the vhf portion of the preamp from the uhf antenna is liable to be much greater than 3db because of the L/C (coil and capacitor) filters for each band. Ideally designed, the L/C filter should cut ALL the signal from outside of its frequency range 100%. But in real life its never 100% so some UHF signal will get thru, as will all the VHF signal on that VHF preamp input.

Modifying the preamp is a possibility, but without having high frequency RF test equipment, chances of success are close to nil.

The 3db loss I mentioned does come from the combiner/antenna in a different direction, which youll have to use on the one uhf preamp input, in that scenario.

There are two possibilities:
- separate UHF and VHF antennas with separate feeds to the pre-amp and no combiner loss
- separate UHF and VHF antennas combined at the antenna before feeding to the pre-amp, suffering a -3dB combiner loss

The OP is confused by your responses because, as I read it, he's got the first case but you keep giving him the second answer. He has no UHF antenna input to the VHF pre-amp, so your response doesn't apply.

Frank
post #633 of 4787
Quote:
Originally Posted by rmcneil321 View Post

So to clarify

Scenario 1:
Single compromise antenna receiving RF9 through RF48 connected to a single input preamp designed to pass both VHF and UHF such as CM0064.

Scenario 2:
Antenna 1 targeted at RF9 and attached to VHF input of 2 input preamp such as CM0264.
Antenna 2 targeted at RF18 to RF48 attached to UHF input of the same preamp as Antenna 1.

So Scenario 1 would actually have less signal loss because it is not filtering anything and then recombining? That does seem logical.

How do the 2 scenarios affect the Signal to Noise ratio? Would the S/N for both be the same, just a lower output in Scenario 2, or do components other than the first transistor add more noise?

Thanks


The 0064 and the 0264 use the same circuit board the only difference being that the input filters are different and there are added ones for VHF. Think of the 0264 as 0064 with a built in UHF/VHF combiner while the 0064 just passes everything ch2-83.
The same thing could be accomplished by using a UHF/VHF combiner ahead of the 0064 but why bother just buy a 0264.

Both should have the same signal to noise ratio since they both use the same amplifier circuit.

As far as signal loss goes, (not counting the different antennas part of the scenarios) either amps input filters will pass the designed frequencies to the amp with very little loss so that should be nearly the same too.
post #634 of 4787
I guess for my situation, instead of trying to predetermine if I will ultimately want an 0064 or a 0264, I should probably just get a 7777 or 7778. Then I can do testing with single antenna and two antennas (antennae?) and see which works best and just leave it whichever way works best. Now I just have to decide if I really need the extra boost of the 7777 or if that is just going to cause more problems which will require attenuation. I'm probably better off with the 7778 since my current signal path is ~25ft directly into a CECB. Once I get my antenna sorted I will be splitting to a second CECB. Then later if we add more signal suckers (real DTV's) to the system I can add a good distribution amp downstream.

I figured by going with the Spartan series I could eliminate the balun, but if I put a good balun on, it should essentially be a wash between my balun and the internal 300:75 transformer on the 0064/0264. Are the CM baluns worthy? To get up and running I have a $2 Phillips balun on my initial testbed antenna.

Can of worms anybody? Anybody want to pass judgment on Solid Signal vs Summit Source?
post #635 of 4787
Quote:
Originally Posted by rmcneil321 View Post

I guess for my situation, instead of trying to predetermine if I will ultimately want an 0064 or a 0264, I should probably just get a 7777 or 7778. Then I can do testing with single antenna and two antennas (antennae?) and see which works best and just leave it whichever way works best. Now I just have to decide if I really need the extra boost of the 7777 or if that is just going to cause more problems which will require attenuation. I'm probably better off with the 7778 since my current signal path is ~25ft directly into a CECB. Once I get my antenna sorted I will be splitting to a second CECB. Then later if we add more signal suckers (real DTV's) to the system I can add a good distribution amp downstream.

I figured by going with the Spartan series I could eliminate the balun, but if I put a good balun on, it should essentially be a wash between my balun and the internal 300:75 transformer on the 0064/0264. Are the CM baluns worthy? To get up and running I have a $2 Phillips balun on my initial testbed antenna.

Can of worms anybody? Anybody want to pass judgment on Solid Signal vs Summit Source?


Unless you have some really strong local signals (including FM Radio) the 7777 should be OK. A lot of people really like them and the added gain could help avoid a distro amp if you start doing a lot of splitting.

I use the 0064 because then I can vary the length of the twinlead to further tune the input for my homemade antennas if needed and avoid the balun loss, plus I was getting them dirt cheap $20 and already had the twinlead.

I also think that amping in smaller steps as needed is better than one big step at the beginning. I may be all wet on that but it's working for me.

Some people would say that for a run of 25' that an amplifier is not needed. Overloading and high urban noise problems aside, I think that an amp with a noise figure smaller than your receiver will always improve your reception and there are very few if any receivers that have a noise figure as low as 3 db.
post #636 of 4787
Quote:
Originally Posted by MikeySoft View Post

Wouldn't having a 90 degree angle between the two phase lines when they cross minimize any interference between the two? Why is it recommended to keep a space between them?

I still don't understand this unless the phase lines are like a 300 ohm cable where you need to keep the separation constant. I don't understand 300 ohm cable either.
post #637 of 4787
Here is what I ended up with.....

9 1/2" whiskers and 9" spacing center to center. 1/8" thick Neoprene washers used to isolate the elements and feedline from the 2x6.



post #638 of 4787
Quote:
Originally Posted by MikeySoft View Post

I still don't understand this unless the phase lines are like a 300 ohm cable where you need to keep the separation constant. I don’t understand 300 ohm cable either.

Actually you do. The trick is to keep wires of certain gauge a constant distance apart. If you want 600 ohm, the distance is larger for the same gauge. Encasing in plastic changes the equation.
http://www.qsl.net/co8tw/openline.htm

Quote:


Here is what I ended up with.....

9 1/2" whiskers and 9" spacing center to center. 1/8" thick Neoprene washers used to isolate the elements and feedline from the 2x6.

Looks OK EscapeVelocity, but you should have asked for wire straightening tips before you started. The easiest way to get a perfectly straight wire is to clamp one end in a vice, and the other end in vice grips (or twisted around some other pliers). Then hit the vice grips hard with a hammer a few times. It does a great job on making wire (smaller than 4 gauge) straight and smooth. It is possible that crooked wire bent a random way outperforms a perfectly straight wire, but the law of averages is way against you.

Quote:


So to clarify

Scenario 1:
Single compromise antenna receiving RF9 through RF48 connected to a single input preamp designed to pass both VHF and UHF such as CM0064.

Scenario 2:
Antenna 1 targeted at RF9 and attached to VHF input of 2 input preamp such as CM0264.
Antenna 2 targeted at RF18 to RF48 attached to UHF input of the same preamp as Antenna 1.

So Scenario 1 would actually have less signal loss because it is not filtering anything and then recombining? That does seem logical.

There is a scenario 3, buy another preamp, 1 preamp per antenna, and combine the 2 after amplification with a combiner/splitter that lets the signal go both ways so power can get to both amps. The transistor in the 0064/0264 is spec'd at drawing only 100 ma maximum. The 0747 power injector looks like its outputing at least 400ma, so it shouldnt have any problems powering two preamps.

Actually, if you can wait until after this weekend, I can test to see just how much uhf signal gets thru on the 00264 preamp vhf input.

Quote:


Are the CM baluns worthy? To get up and running I have a $2 Phillips balun on my initial testbed antenna.

Can of worms anybody? Anybody want to pass judgment on Solid Signal vs Summit Source?

CM baluns are probably the best, but the Phillips is fine if you dont break it. Solid Signal has amazing delivery for the UPS ground price.
post #639 of 4787
Thanks for the tip 300ohm. I could have really used that before I built my latest antenna.

Does that effect the antennas performance?
post #640 of 4787
Quote:
Does that effect the antennas performance?

Hard to tell. The modeling was done with a straight wire, so it deviates from the model. Like I said, there is a chance the way you have them bent could be increasing performance. But the odds are way against it, so its most likely to be hurting performance.

Quote:
The OP is confused by your responses because, as I read it, he's got the first case but you keep giving him the second answer. He has no UHF antenna input to the VHF pre-amp, so your response doesn't apply.
Frank

Frank, the CM00264 has one 300ohm uhf input and one 300ohm vhf input and one 75ohm output. He was asking about using the vhf input as a second uhf input. Ill try that crazy scenario this weekend, as Im also interested as to how efficient that L/C circuit is.
post #641 of 4787
Quote:
Originally Posted by MikeySoft View Post

I still don't understand this unless the phase lines are like a 300 ohm cable where you need to keep the separation constant. I don't understand 300 ohm cable either.

The phase lines are nothing more than a transmission line that connects all the elements together just like 300 ohm twinlead.
The spacing between the wires and the wire size determines the line impedance.

It's like a whole series of series and parallel reactance's working together throughout the line. Those combined reactive elements determine the impedance (ohm) value of the line.

An open wire line has a very high velocity factor which allows us to keep the vertical bay seperation at maximum while still staying in phase.

If we were to use 300 ohm or some other type of feedline between the bays the plastic coating on and between the wire lowers the velocity factor which means that the wire (phase line) would have to be shorter to maintain phasing. That would mean that the vertical bay spacing would have to be smaller to match the shorter phase line. Closer bay spacing doesn't produce as much gain.

If we don't shorten the vertical bay spacing then the phasing line would be too long and the bays would be thrown out of phase and no longer work together as effiecently.

If we don't keep the spacing consistant in our phase line the impedance will vary and throw unaccounted for reactance into the antenna (unless you model it that way).

This could work for you or against you just like anything else that's changed from what was modeled.

I hope I got that all right someone please correct me if I'm wrong
post #642 of 4787
Quote:
Originally Posted by 300ohm View Post

Frank, the CM00264 has one 300ohm uhf input and one 300ohm vhf input and one 75ohm output. He was asking about using the vhf input as a second uhf input. Ill try that crazy scenario this weekend, as Im also interested as to how efficient that L/C circuit is.

Actually my scenario called for using the VHF input for RF9 which is VHF-Hi.
post #643 of 4787
Yes, as for the transmission line between the elements its nothing more than that , and the twist is need because there is about a half wavelength between the top element and the one below.and you want the current flowing the same on all of the elements.. Get it? :P


Think of big twin lead with no plastic LOL. :P

JOhn
post #644 of 4787
What can I do to improve my latest creation?

Or is it pretty good, as is?
post #645 of 4787
Keeping the phase line at a constant distance at the cossing increases the length. But I guess, keeping the impedance at 300 ohms is more important.
post #646 of 4787
Quote:
Originally Posted by 300ohm View Post

The easiest way to get a perfectly straight wire is to clamp one end in a vice, and the other end in vice grips (or twisted around some other pliers). Then hit the vice grips hard with a hammer a few times. It does a great job on making wire (smaller than 4 gauge) straight and smooth.

Copper is very soft. Will they stretch and become thinner using this technique?
post #647 of 4787
Quote:
Originally Posted by MikeySoft View Post

Wouldn't having a 90 degree angle between the two phase lines when they cross minimize any interference between the two? Why is it recommended to keep a space between them?

You aren't trying to "minimize interference". Indeed, specific interactions between the two wires are desired. The parallel wires form a transmission line. Transmission lines have near magical powers. They require constant spacing. Any discontinuity tends to result in reflections and other weird effects that can sometimes be beneficial but are usually detrimental if they weren't deliberate.

Some examples of transmission lines:
  • Open air balanced transmission lines (as used here)
  • Twin lead
  • Coax
  • Ethernet, USB, firewire, RS-422/485, etc.
  • High speed signals connecting components on PC motherboards
Two parallel wires, with or without a ground plane, or one wire with a ground plane or shield make a transmission line if they are long relative to the highest frequency in use. If the distance is shorter than 1/6 of a wavelength, they can be treated as a lumped sum capacitance for digital signals but longer than that and they are a transmission line; for analog signals, effects may be significant even at 1/6 wavelength.

A short wire, relative to the wavelength, has, more or less, the same voltage at every point on a wire. For a transmission line, the signal travels down the wire and different voltages are simultaneously present at different points on the wire. If you send a +/-1V signal down a transmission line one wavelength long (with no reflections or other oddities) you will have +1V at one point on the wire and -1V on another and every voltage in between simultaneously.

Imagine a pond or other still body of water. Now draw an imaginary line segment across the surface of the pond with two endpoints. Drop a pebble at one end and watch how the waves travel down that line. If you put a large solid object in the water along the line, you can see reflections occurring that will act somewhat like reflections from discontinuities in the line. If you imagine that the line passes through two fluids with different viscosities, say water and oil that have magically kept separate sideways instead of the usual vertical separation, that is roughly equivalent to having two different impedances along the line. The wave energy from the water doesn't smoothly transfer to the oil (some does, some doesn't); the energy that doesn't transfer is reflected.

Oversized washers on the line act as a capacitor on the line and create an impedance mismatch that can cause reflections. At certain key points (i.e. where the antenna elements connect and at the feedpoint), the capacitance could be good or bad, depending on whether it improves the impedance match of the elements at that point or makes the match worse. Crossing the wires makes a portion of the impedance line have much lower impedance which creates a discontinuity. This is usually bad.

A transmission line generally wants to have a constant impedance along the entire length of the line (which means, among other things, constant spacing and the same interactions with external conductors or dielectrics along the entire length), be driven by a source that matches the characteristic impedance of the line and be terminated by a resistor that matches the characteristic impedance. Any exception causes trouble unless you are specifically exploiting the specific weirdness that results to your benefit.

Avoid any non-uniformities in transmission lines, including non-uniformities in objects in their vicinity. Do not change any parameter of a transmission line including: wire diameter, wire spacing, wire length, wire insulation, metallic objects in their immediate vicinity, and non-conductive objects in their immediate vicinity. Immediate vicinity would be several times the spacing. An exception is shielded cables, such as coax and twinax, which tolerate objects in their vicinity. Another other exception is irregularities deliberately introduced by someone who understands transmission lines. If a transmission line is driven with its characteristic impedance (usually not true at most frequencies on an broadband antenna, unfortunately) and is also terminated in its characteristic impedance, then the length of the line has little effect except for some loss proportional to length. Otherwise, you have frequency dependent effects that change with the length of the wire. This is one reason an amplifier at the antenna can improve things because the coax cable is driven properly at all frequencies.

TC 9-64 (link posted earlier) has a description of how a transmission line works.

As a random example of the wierdness of transmission lines, I once had some network cards that had suffered lightning damage on a long bus topology network. Many hundreds of feet of cable connected computers in one or two buildings (don't remember how many at that time). Reflections were caused by the damaged cards. As a result, two PCs couldn't talk to each other even though the same PCs could talk to other PCs. The reflections from a 3rd (damaged) computer were canceling out the signals those specific PCs saw form each other. A 10volt signal was being annihilated! Took a while to figure that one out.
post #648 of 4787
Quote:
Originally Posted by 300ohm View Post

...
Frank, the CM00264 has one 300ohm uhf input and one 300ohm vhf input and one 75ohm output. He was asking about using the vhf input as a second uhf input. Ill try that crazy scenario this weekend, as Im also interested as to how efficient that L/C circuit is.

Quote:
Originally Posted by rmcneil321 View Post

Actually my scenario called for using the VHF input for RF9 which is VHF-Hi.

300ohm, you may want to test your understanding ... words can be an ambiguous communications medium.
Frank
post #649 of 4787
Quote:
Originally Posted by rmcneil321 View Post

Actually my scenario called for using the VHF input for RF9 which is VHF-Hi.

Heh, then I guess I really got your question confused. Yes, attaching a 300 ohm vhf antenna to the vhf input of the preamp and attaching a 300 ohm uhf antenna to the uhf input of the preamp works great. Thats what the CM 0264 was designed for.
Quote:


Copper is very soft. Will they stretch and become thinner using this technique?

Yes eventually, if you keep beating on it. 12 and 14 gauge copper wire only takes a few medium taps. 6 gauge copper wire takes some serious beating to get straight.

Quote:


What can I do to improve my latest creation?

Or is it pretty good, as is?

Test it.
Quote:


and the twist is need because there is about a half wavelength between the top element and the one below.

At 8 inches, its actually a full wavelength. I also fall into the habit of thinking half wavelength.
Quote:


I once had some network cards that had suffered lightning damage on a long bus topology network.

Heh, I also once had lightning hit close by the underground network cables.
It fried them and fused the soil around them, so it looked like a soil pipe. I was about 100 foot away at the time.
post #650 of 4787
Quote:
Originally Posted by 300ohm View Post

At 8 inches, its actually a full wavelength. I also fall into the habit of thinking half wavelength.

A full wavelength at 600mhz is 19.68 inches (in air).

In open air radio waves travel at the speed of light = 300,000,000 meters per second.


300 divided by the frequency in mhz= wavelength in meters

Wavelength in meters multiplied by 39.37 = wavelength in inches
post #651 of 4787
Quote:
Originally Posted by fbov View Post

300ohm, you may want to test your understanding ... words can be an ambiguous communications medium.
Frank

While I was frustrated by the apparent mis-communication taking place between myself and 300ohm, it actually was beneficial in the long run as I did actually spend some time thinking about what was taking place inside the amps. While it should have been obvious, it hadn't occurred to me that if I just used the UHF input of the 0264 with a single antenna it would filter out my channel 9. I ordered a 7778 (so I can have the switch) and will resume testing next week.
post #652 of 4787
So you actually don't have a VHF input on this pre-amp?
post #653 of 4787
Quote:
Originally Posted by mclapp View Post

You may want to do your tests without the pre-amp sometimes it's easier to tell the small changes. I've used attenuators sometimes just to get the signal down to the threshold so the change shows up better to the eye.

One trick is to put a variable attenuator in the line. Radio shack sold these at one time and you can find them on ebay (make sure you get one suitable for TV coax cable - 75 ohms and F connectors) for about $10 plus shipping. Solid Signal sells a couple for $12/$20. "Calibrate" the knob with repeatable - and preferably meaningful - numbered marking. For example, if you have a 0-20dB attenuator, divide the total rotation of the know into 20 marks (draw a label on your computer and print it). If the attenuator has a logarithmic taper, this will give you 1dB steps - if you are lucky. Or use a 360 degree protractor. But don't rely on accuracy if you don't have a spiffy laboratory grade attenuator. There is probably a reason the knob wasn't calibrated. Rather, use the numbers for comparison with others used on the same attenuator.

Install the antenna on the input to your receiver (as close as possible so defects in attenuator design have minimum effects). Now, rotate the dial slowly until the picture starts to break up and note the setting. Make your changes and repeat until you have the same level of degradation.

Consumer grade attenuators are probably not linear in dB with knob position. Taking the attenuator apart and drawing a schematic as well as using a multimeter to determine the taper of the potentiometer can help understand the amount of attenuation you actually get as well as any impedance mismatch effects (VSWR) that may result. It is possible that VSWR changes might be such that actual attenuation might be non-monotonic on a cheapo attenuator.



Bear in mind that signal levels arriving at the antenna vary with time.

Constructing a variable attenuator such that you maintain the proper impedance on both sides is tricky. A unit with 24dB, 12dB, 6dB, 3dB switched attenuator sections or a step attenuator is easier. A continuously variable attenuator would appear to require a custom potentiometer with 1 log taper and 2 antilog tapers, and log tapers often aren't really. Making a programmable gain amplifier or buffered attenuator is likely to be easier as you don't have the impedance matching issues.

http://www.electronics-tutorials.com...ttenuators.htm
http://sound.westhost.com/pots.htm#taper

S-meters on ham gear that can tune the VHF/UHF TV bands may be of limited use. In theory, the S-units on the signal strength meter correspond to 50uV at S9 with 6dB steps down from there for each s-unit. Even though this standard is listed in the ARRL handbook and is in the ham radio exam question pool, rigs that even come close to adhering to it are the exception rather than the rule. And some units with LCD displays only have 1 step per S-unit which gives you only 6dB resolution if it uses 6dB/unit.
http://www.ac6v.com/sunit.htm

My yaesu VX-7R shows S-9 on one of the analog TV channels, even though the audio quality is pretty poor.
post #654 of 4787
Quote:
Originally Posted by fbov View Post

So you actually don't have a VHF input on this pre-amp?

What I don't actually have is a pre-amp. I was thinking out loud about what pre-amp to get for testing since I won't know until after testing what my pre-amp needs will ultimately be.
post #655 of 4787
Quote:


A full wavelength at 600mhz is 19.68 inches (in air).

Oops, screwed up my math again.

Quote:


While I was frustrated by the apparent mis-communication taking place between myself and 300ohm, it actually was beneficial in the long run as I did actually spend some time thinking about what was taking place inside the amps.

Glad I could help.


On another note, I downloaded the CM4221a.ez file from http://hometown.aol.com/kq6qv/SIMS/ and loaded it into my 4nec2x program. I all did to it was close the ends of the whiskers with a 1 inch indentation (so the elements look like the classic bow-tie) and got a 2.9 db increase in raw gain at 470 mhz (the low end of channel 14) and a 2.1 db increase at 698 mhz (the high end of channel 51). Combined with mclapps large curved reflector, that should be one hot antenna. Standard CM4221's could be retrofitted with crimp on connectors.
post #656 of 4787
Quote:
Originally Posted by 300ohm View Post


On another note, I downloaded the CM4221a.ez file from http://hometown.aol.com/kq6qv/SIMS/ and loaded it into my 4nec2x program. I all did to it was close the ends of the whiskers with a 1 inch indentation (so the elements look like the classic bow-tie) and got a 2.9 db increase in raw gain at 470 mhz (the low end of channel 14) and a 2.1 db increase at 698 mhz (the high end of channel 51). Combined with mclapps large curved reflector, that should be one hot antenna. Standard CM4221's could be retrofitted with crimp on connectors.

It would be super, a nice mod for a 4221 and also to make a stronger DIY antenna.

Be sure to do a AGT test on it, that's a pretty big jump.

I don't remember getting that kind of gain when I tried that, but I never tried it on a 4221, only on my own models.
post #657 of 4787
Quote:


Be sure to do a AGT test on it, that's a pretty big jump.

Thanks for the tip. Hmmm, at 470mhz the AGT results show 1.61 (2.06db) and at 698mhz the AGT results show .68 (-1.7db).

The 4nec2 help file under AGT:

"The following gradation can be made for the Average-Gain-Test results:

> 0.95 and < 1.05 Model is likely to be accurate
> 0.90 and < 1.10 Model is usable for most purposes.
> 0.80 and < 1.20 Model may be useful, but can be improved.

< 0.80 or > 1.20 Model is questionable and should be refined.

When the resulting value is below 0.8 or above 1.2 the value is displayed in red color.

The db value between brackets can be used to correct the far-field pattern gain."

So would that mean 4.96 db additional gain at channel 14 and only .4 db additional gain at channel 51 ?

(I also ran the AGT test on the original CM4221a.nec file and got at 470mz 1.32 (1.19db) and at 698mhz .46 (-3.4db) )
post #658 of 4787
Quote:
Originally Posted by mclapp View Post

A full wavelength at 600mhz is 19.68 inches (in air).

For quick reference:

VHF Low 54-88Mhz 218.57 down to 123.12 inches
VHF high 174-216Mhz 67.83 down to 54.64 inches
UHF post transition: 470-698Mhz 25.11 down to 16.91 inches
Top of channel 69 : 806Mhz 14.64 inches
Top of Channel 80: 890Mhz 13.26 inches

Center of UHF band, post transition (between channels 32 and 33):
584Mhz 20.21 inches

Center of VHF Low: 71Mhz 166.24 inches
Center of VHF High: 195Mhz 60.53 inches
post #659 of 4787
Quote:
Originally Posted by 300ohm View Post

Oops, screwed up my math again.


Glad I could help.


On another note, I downloaded the CM4221a.ez file from http://hometown.aol.com/kq6qv/SIMS/ and loaded it into my 4nec2x program. I all did to it was close the ends of the whiskers with a 1 inch indentation (so the elements look like the classic bow-tie) and got a 2.9 db increase in raw gain at 470 mhz (the low end of channel 14) and a 2.1 db increase at 698 mhz (the high end of channel 51). Combined with mclapps large curved reflector, that should be one hot antenna. Standard CM4221's could be retrofitted with crimp on connectors.

Nice experiment, been thinking about doing something along those lines myself after I have a chance to upgrade the code to facilitate optimization. The 4221 elements, as I recall, were pretty undersized. Some of the better homebrew dimensions here might not see as much improvement, though 2.1dB at 698mHz suggests that they might still get some significant benefit. Also have to look at net gain, which I am not sure 4nec2 can calculate automatically but we discussed how to calculate it here earlier.

The improvement does seem a bit high. I doubt the AGT test would fail provided you used the same size wire, though. And it is only around 0.525dB to 0.725dB per bowtie (half that if you consider that the wavefront hits each bowtie twice due to the reflector) which makes it more plausible.

You might try changing the dimple from an innie to an outie. Some biconic antennas do this. Might improve the low end, not so sure about the high end. But it is the low end that needs it the most.
post #660 of 4787
Quote:
Originally Posted by 300ohm View Post

Thanks for the tip. Hmmm, at 470mhz the AGT results show 1.61 (2.06db) and at 698mhz the AGT results show .68 (-1.7db).

The 4nec2 help file under AGT:

"The following gradation can be made for the Average-Gain-Test results:

> 0.95 and < 1.05 Model is likely to be accurate
> 0.90 and < 1.10 Model is usable for most purposes.
> 0.80 and < 1.20 Model may be useful, but can be improved.

< 0.80 or > 1.20 Model is questionable and should be refined.

When the resulting value is below 0.8 or above 1.2 the value is displayed in red color.

The db value between brackets can be used to correct the far-field pattern gain."

So would that mean 4.96 db additional gain at channel 14 and only .4 db additional gain at channel 51 ?

(I also ran the AGT test on the original CM4221a.nec file and got at 470mz 1.32 (1.19db) and at 698mhz .46 (-3.4db) )

If you get a positive AGT gain it means that the model is showing you that much gain above what it should so you have to subtract that number from the gain figure. Of course a negitive number works just the opposite.

That's why it's difficult to just run a sweep of the whole band and get good data. Because of the reactive nature of these antennas the source has to be adjusted quite often across the UHF band and to keep the numbers close.
It's even worse when running models of UHF bowties o VHF-HI.


A quick way to fix the model so that you get the AGT near 1.00 is to change the diameter of the wire the source is located on. If your getting a positive reading make the wire slightly smaller.
If the AGT is too far off it will also throw off the impedance data as well.

Have Fun!
New Posts  All Forums:Forum Nav:
  Return Home
  Back to Forum: HDTV Technical
AVS › AVS Forum › HDTV › HDTV Technical › How to build a UHF antenna...