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How to build a UHF antenna... - Page 133

post #3961 of 4789
Is the SWR important if you only are receiving with the antenna and not transmitting?
Reason is that I tried to simulate a 2 bay, but instead of whiskers I just used 1 wire per whisker. Gain looks good but SWR increases up to approx 40 at high frequencies. See picture and nec file. (I hope i did ok with the simulation)
What level should the SWR normally be and what are the downsides of high SWR?
LL

 

2-Bay_8.5x8.5_BowSwp=0.0in_NO_Refl_FeedSep=1.5in_Diam=1mm.BowSep=0_4w.nec.doc 3.8134765625k . file
post #3962 of 4789
SWR has the same ill effects to the received signal.
post #3963 of 4789
Quote:


Reason is that I tried to simulate a 2 bay, but instead of whiskers I just used 1 wire per whisker. Gain looks good but SWR increases up to approx 40 at high frequencies.

The higher the SWR, the higher the mismatch loss, the less the NET gain. For example, at SWR = 2.0 the mismatch loss is .51 db. At SWR = 4.0 the mismatch loss is 1.94. At SWR 40, the mismatch loss is extremely high, making the antenna ineffective.
post #3964 of 4789
300 ohm, do you have a formula for converting raw(total) gain + SWR to net gain? Several modelers show raw gain and SWR but don't provide net gain.

Thanks!
post #3965 of 4789
Hi folks, new to this section. I can only put an antenna in the attic, and i am about 41 miles from signal (ESB in NYC). I have a bunch of trees too. Here is my TvFool page.

http://www.tvfool.com/?option=com_wr...cd7225fee905ae

Can someone tell me if a 4 bay might work, or are the hoverman best for in the attic? Any guidance appreciated. thanks
post #3966 of 4789
Quote:


300 ohm, do you have a formula for converting raw(total) gain + SWR to net gain? Several modelers show raw gain and SWR but don't provide net gain.

Here is a method to calculate Net Gain:

NetGain = RawGain+10*log(Feed-pointGain)
where Feed-pointGain = 4*Zr*Zo/((Zr+Zo)^2+Zi^2)

The [10*log(base10)] converts to decibels, and Feed-point Gain is less than 1, since there is a loss.

Zo = characteristic Impedance for the transmission line connected to the antenna (For use with 300 ohm twin lead or a 4:1 balun, Zo = 300)
Zr = real part of antenna's complex impedance at a specific frequency
Zi = imaginary part of antenna's complex impedance at a specific frequency
RawGain = Gain output given by the NEC2 program at the specific frequency

A quick way to look at the SWR figures to subtract from Raw Gain :
Code:
SWR     Mismatch Loss dBi
-----   -----------------
1.0     0
1.5     .18
2.0     .51
2.5     .88
3.0     1.25
3.5     1.60
4.0     1.94
4.5     2.25
5.0     2.55
Over SWR = 5, the antenna is so poor as to be basically unusable, as 3 dbi is half the signal. But under certain unusual circumstances, Raw Gain can be more important than Net Gain.
post #3967 of 4789
Quote:


I can only put an antenna in the attic, and i am about 41 miles from signal (ESB in NYC). I have a bunch of trees too.

Ewww. In the attic, youll be lucky to get a couple of stations.
post #3968 of 4789
Thanks 300ohm, great explanation and very helpful table!

Best,

Rick
post #3969 of 4789
Quote:
Originally Posted by MBS View Post

Is the SWR important if you only are receiving with the antenna and not transmitting?
Reason is that I tried to simulate a 2 bay, but instead of whiskers I just used 1 wire per whisker. Gain looks good but SWR increases up to approx 40 at high frequencies. See picture and nec file. (I hope i did ok with the simulation)
What level should the SWR normally be and what are the downsides of high SWR?

Net Gain attempts to measure the depth of the standing wave nulls, which for each channel,
will be located at different lengths along the coax. Hence it may or may not be a problem,
and could be "moved" elsewhere by inserting short lengths of additional coax.
[Murphy's Law predicts it will be worst on your weakest stations.]

And finally, and of utmost importance for DIGITAL, is the additional degradation
caused by the reflections up and down the coax looking like short delay multipath,
the worst kind for the equalizer.
As mentioned previously in this thread, EVM (Error Vector Magnitude) is a measurement
of how the digital "eye pattern" is degraded by SWR reflections:
http://www.avsforum.com/avs-vb/showt...php?p=18446671
http://www.avsforum.com/avs-vb/showt...php?p=16681206

So try to keep SWR under 2.0 and there could be significant degradation at 3.0.
post #3970 of 4789
Thanks 300Ohm, I have heard other folks have success in the attic, but maybe they were not as far. So, no matter what I build, at that distance it is not likely to get much signal? I thought I saw somewhere that the GH were good in the attic. But I dont know a lot about this.
post #3971 of 4789
Quote:
Originally Posted by sabre6 View Post

Thanks 300Ohm, I have heard other folks have success in the attic, but maybe they were not as far. So, no matter what I build, at that distance it is not likely to get much signal? I thought I saw somewhere that the GH were good in the attic. But I dont know a lot about this.

A) I have a similar signal levels in my area. You would be lucky to get anything in the pink in the attic. It's hard enough to get them outdoors.

B) You have 3 Hi-VHF stations. Some will claim you can modify GH and 4-Bay to get in Hi-VHF. But I don't think so, not at those low levels. The only thing that worked for me is a yagi.
post #3972 of 4789
Thanks BCF68. As you have similar levels near you, what do you use to pull them in, and where did you place it?
post #3973 of 4789
sabre6,
As others have said in the attic I would also say you have slim to no chance of getting reliable reception with almost anything you could buy or build if the TVFool report is correct.

If you were to build a 4 bay I would suggest a 9" whisker with 8 1/2" bay spacing. That would give you the best shot of VHF ch's 11 and 13 and most of your UHF is above ch30 so it will be good there as well. To have a real shot you would need to mount it outdoors and install a quality amp like cm7777.

My area also has tvfool numbers very similar to yours in fact in some places I've installed them even worse.

Unlike others I have had good success getting VHF-hi stations down into the single numbers on the tvfool chart using a properly sized and built 4 bay with a wide screen reflector. It may have more to do with the accuracy of the tvfool projection than anything. A proper sized 4 bay will have decent positive gain (about 7 db) on a small range of channels for VHF-hi depending on size. If you have a positive noise margin, the antenna has some gain and the amp takes care of the down stream losses then you should get the station, but when you're on the edge and that prediction is a little off you may not get enough gain to overcome the difference.

If the 4 bay doesn't get the VHF-hi stations you want then you could add a seperate VHF-hi antenna later and the cm7777 has a seperate vhf port just for that set-up.
post #3974 of 4789
Thanks Mclapp. I will have to try to get this outside. Do you think i could get them outside w/ no reflector?
post #3975 of 4789
I think you're going to need the reflector especially for any shot of VHF-hi reception with that style antenna.
post #3976 of 4789
Just out of curiosity, has anyone experimented much with other iterations of the 1+3x configuration on these antennae? mclapp, I've seen your vertical 10-bay, and I imagine anything bigger than that would get unwieldy. 7-bay, anyone?
post #3977 of 4789
The biggest I've gone is a vertical stack 8 bay, some have stacked 4228's and similar which would be quite large as well.
post #3978 of 4789
Sure, but multiples of 4 (excluding 4 itself) are somewhat unnatural for this design. That's why you have to add leads to link two 1+3's. So, rather than just linking multiple 4-bay antennas, why not just continue the phase lines on out to 7 or 10 bays?
post #3979 of 4789
I built a 6 bay once all using the same phase line. It really didn't work any better than the 4 bay overall and the VHF-hi suffered. Going more than 4 starts to run into out of phasing problems on the outer elements.

Most ham radio antenna diagrams I've seen of this type are 3 bays max then they stack the 3 bays if they want more.
post #3980 of 4789
I see. I was just curious as I was dismantling the horizontal 8 this weekend and trying to imagine what other configurations might be possible. It just seemed that the mathematically logical thing to do would be to add bays in multiple of 3 to the base 4-bay, which in itself might be viewed as a 1-bay with 3 bays added on.

Personally, I've had about enough stacking. I'm going back to the old 4-bay, with a wider, angled reflector. This one will be 52" wide with a more precise angle right in the middle. Forward swept and amplified, whatever I get is what I get. A ladder should never be a permanent fixture on the back deck. It gets in the way of the grill, and it makes a lowsy place to set a beer.
post #3981 of 4789
How far away do whiskers/phase lines need to be away from wood to prevent any interference? I do not have any part of the 4-bay touching the wood, but it might only be off the wood by 1/8 - 1/16 of an inch
post #3982 of 4789
sustorm,
The key is to keep contact of the elements and phase lines to any objects to a minimum especially in the junction areas. Wood will be one of the worst as far as detuning goes because of the water content it can retain.

That being said any air gap at all even 1/16th of an inch should be enough for poor electrical conductors like wood. For conducting materials like metal a bigger gap is usually needed, it appears 1/2" or more is better.

jstarling82,
I've said it many times, a well built 4 bay is the way to go for most situations, I would take a 4 bay and the proper amp over a 8 bay stacked most every time. Don't get me wrong an 8 bay will work better in many situations but the difference isn't that much. It can take a lot more tuning, tweeking and positioning to get it right. Unless you have a great big tower that's out in the clear to stick it on or the right equipment to dial it in many won't get the full advantage of the larger stacked antenna gain wise.

Most people would find bigger gains in thier distribution system and antenna positioning than the 4 bay to 8 bay gains.
post #3983 of 4789
Quote:
Originally Posted by mclapp View Post

jstarling82,
I've said it many times, a well built 4 bay is the way to go for most situations, I would take a 4 bay and the proper amp over a 8 bay stacked most every time. Don't get me wrong an 8 bay will work better in many situations but the difference isn't that much. It can take a lot more tuning, tweeking and positioning to get it right. Unless you have a great big tower that's out in the clear to stick it on or the right equipment to dial it in many won't get the full advantage of the larger stacked antenna gain wise.

Most people would find bigger gains in thier distribution system and antenna positioning than the 4 bay to 8 bay gains.

Well said. Sometimes, though, I like a bit of personal experience to really drive the point through my thick skull.

With the horizontal stack, I did notice enough gain to make a difference, particularly without the amp in place. However, multipath interference (I assume) that was introduced into the system by this setup negated the benefits of the increased gain. Sure, I was able to tune a few extra channels, but the antenna was so picky, I could only channel surf with someone on the roof to make adjustments, and the additional channels I picked up weren't worth losing a steady picture on the others.

I'd still love to do some experimentation with additional iterations of the 1+3x configuration, but for practicality's sake, I'm satisfied with the classic.

I'll be sure to post pics when I get the 4-bay put back together. It's come a long way, and I'll definitely need some input. Particularly, I'm using 4 horizontal pieces of emt for stability. These pieces run in an odd pattern behind the antenna, but about an inch in front of the curved reflector. My thought is that there isn't enough there to cause a problem, but I wonder what I'll actually find to be the case.
post #3984 of 4789
I recently built the 4-bay, 9" whiskers with 8 1/2" spacing antenna based on mclapp's design and it works very well for me.
I'm in the Houston suburbs (77346 zip code), and luckily all my major towers are located in the same spot, so once I aim the antenna properly, I'm good to go.
The major networks are both in the UHF range (NBC, FOX, CW, MyTV) and Hi-VHF (ABC, CBS, PBS), so mclapp's antenna, based on the simulation tests was a good choice.
I built it without a reflector, and installed it on my fence, about 7 to 8 feet high and pretty much got all the UHF stations showing 100% strength on my DISH vip722 box.The Hi-VHF stations were in the 75-85% range, but unfortunately, I'd get occasional breakups (down to 0% )
I was planning on getting some material to put a reflector to see if that would help a bit, but a funny thing happened. My coax was only loosely running on the ground out to the grounding block since I wasn't sure whether this would become a permanent installation, so I decided to tidy it up a bit. All of a sudden, I was also getting 100% strength on the Hi-VHF channels with no breakup
I played a bit with the cable location to see how it affected the Hi-VHF signal, but I'm not sure if I figured it out. I can move the cable a bit at the anntenna, and have the strength back down to the 75% range, and then move it just right again to get back to 100%.

Is this due to the position of the balun, in relation to the phase lines? Is there a best position for it? I seem to get 100% when the balun is a little bit away from the antenna, and seem to get the signal degradation if I have it lay flat.
Or is this due to the 75 ohm coax itself? This only affects the Hi-VHF stations, the UHF ones remain rock solid regardless.

Any best practices for installing the balun and running the cable? Should it affect it that much, or do I have a bad cable?

Thanks for the help, and special shout-out to Mclapp for an AWESOME design!

CFC
post #3985 of 4789
objects near an antenna can affect its performance, metal objects to a greater degree. you can have both beneficial and diminishing effects. you need to find what works for your particular situation.

sometimes the cable positioning is dictated by physical constraints of where it can be secured. the least effect may be routing the cable as directly away from the antenna as possible and at a right angle to the antenna elements, though you are constrained by where you can secure it.
post #3986 of 4789
Just your touching the balun or getting near the phase lines and elements can change the antenna systems impedance.
That type of antenna is very touchy on VHF-hi the impedances swing a lot with in a few channels so it won't take much to make a change. I guess as already suggested place the balun where it works best for you.

Your pixelation could be from noise and not loss of signal strength, VHF-hi is very sensitive to man made noise like car or lawn mower ignitions, exessive power line noise and other stuff. Of course the stronger the signal the less likely the noise will bother it.
post #3987 of 4789
Thanks, I'll clean up the wiring and shorten my coax run for a permanent installation and arrange it in such a way I get the best signal to the house.

Mclapp: you really should put your website address in you signature line or something I can't stress how much I appreciated the info you got in there.

I appreciate the help,
CFC
post #3988 of 4789
Quote:
Originally Posted by CFC View Post

I recently built the 4-bay, 9" whiskers with 8 1/2" spacing antenna based on mclapp's design and it works very well for me.

Bad link with one extra http; should be:
http://m4antenna.eastmasonvilleweather.com/
post #3989 of 4789
Quote:
Originally Posted by rabbit73 View Post

Bad link with one extra http; should be:
http://m4antenna.eastmasonvilleweather.com/

Thanks ... fixed it

CFC
post #3990 of 4789
Quote:
Originally Posted by CFC View Post

Mclapp: you really should put your website address in you signature line or something I can't stress how much I appreciated the info you got in there.

I appreciate the help,
CFC

I hesitate to do that since there is stuff that people can purchase there along with the info. The stuff for sale could be a violation of this forum so I stay away from pointing to the site except for certain informational pages that may apply to a discussion.
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