Warning: Creating default object from empty value in /homepages/11/d132647312/htdocs/Amfone/mkportal/include/SMF/smf_out.php on line 47
resonant antennas and open wire line




 
The AM Forum
July 30, 2015, 10:04:16 AM *
Welcome, Guest. Please login or register.

Login with username, password and session length
 
   Home   Help Calendar Links Staff List Gallery Login Register  
Pages: 1 [2] 3   Go Down
  Print  
Author Topic: resonant antennas and open wire line  (Read 15721 times)
0 Members and 1 Guest are viewing this topic.
KX5JT
Contributing
Member
*
Offline Offline

Posts: 1919


John-O-Phonic


« Reply #25 on: December 28, 2009, 01:16:04 AM »

I suppose my experience with the G5RV is atypical.  Once I put a W2DU 1:1 Choke Balun where the coax transistions to 450 ohm windowline, all of my RFI and tuning issues dissappeared.  I can tune it 160 - 6 meters flat.   

Yet, I still wanna go with full sized doublet and a balanced tuner.  I just feel like it'll play better.
Logged

AMI#1684
Steve - WB3HUZ
Guest
« Reply #26 on: December 28, 2009, 01:51:22 AM »

John, your situation is not atypical. I hear hundreds of G5RVs on the air and most have good signals. The ones who don't nearly always have them mounted at a very low height. In reality, they are indistinguishable from a dipole at the same height on all bands with the possible exception of 80 meters. Even on 80 meters, if low loss coax is used, the G5RV will only be slightly down. On the higher bands, they will have some gain compared to a dipole - after all Varney designed the antenna to produce a cloverleaf gain pattern on 20 meters, thus the 1.5 wavelength dimension for the flat-top portion.

There is nothing magic about them, either good or bad. That said, most people would probably be better served just running the ladder-line into their shack and matching it with a good tuner. Or they could just change the length of the feedline appropriately.

http://www.w5dxp.com/notuner.htm


For some non-audiophool type info on the G5RV, see the links below.

http://www.w8ji.com/g5rv_facts.htm
http://vk1od.net/antenna/G5RV/index.htm
http://vk1od.net/antenna/G5RV/optimising.htm
Logged
Steve - WB3HUZ
Guest
« Reply #27 on: December 28, 2009, 02:07:09 AM »

LOL. Someone appears to be clueless about modeling or what dBi means.


Since certain touchy feely types like to base their claims on hearsay or antenna computer simulators check out the G5RV simulator screenshot attached below.

Notice the matching section's vertical polarization on both 40 and 80 meters?

 Grin Grin Grin Grin Grin Grin Grin Grin Grin Grin Grin Grin Grin Grin Grin Grin Grin Grin


Er uh. Sorry I don't do any modeling myself just lots of real antennas. When I want something modeled I ask da boys to do it for me.

Am I reading your screenshot correctly or are you stating that -73 dBi represents vertical polarization?
Logged
KX5JT
Contributing
Member
*
Offline Offline

Posts: 1919


John-O-Phonic


« Reply #28 on: December 28, 2009, 02:23:02 AM »

W8JI speaks some real truth about the G5RV!!  It really has a bad reputation that is often undeserved.  I've had on more than one occasion recieved a very nice signal report from somebody then I said my antenna was a G5RV and suddenly they recanted and my signal wasn't "as good as it should be", blah blah blah.....

Amazing.... if the G5RV is magical at all, it's that once someone hears that it's a G5RV it suddenly becomes 2 s units lower than it was!  It plays well on 40 and 20, very well.  I play on 80 meters with 50 to 100 watt carriers and seem to get great reports from other stations... especially if I never mention the antenna.  Grin

I heard someone the other day describe their antenna as a "102 foot doublet".   Heh, I chuckled knowing he had a G5RV and got tired of hearing the "detractors" put it down.



Logged

AMI#1684
Steve - WB3HUZ
Guest
« Reply #29 on: December 28, 2009, 02:35:11 AM »

Yes indeed John. The G5RV has gotten a bad reputation. In some cases, it may be deserved. I've seen some versions that had the cheapest ladder line, a very small balun (probably lossy) and RG-58 as the coax. If you look at VK1ODs info, you can see the RG58 version shows the most loss - not really a big surprise.

Also, the G5RV is often the first antenna used by many new hams and it usually gets put up at rather low heights. Hey, a full-sized dipole will suck when it's only 10-20 feet off the ground and used on 80 meters.   Grin

You did the right thing by putting a good balun on yours.
Logged
Steve - WB3HUZ
Guest
« Reply #30 on: December 28, 2009, 03:19:41 AM »

-73 dBi is effectively zero. It's a purely mathematical number produced by the modeling software. It does not represent anything real. Math and physics are not the same thing. The fact that you don't understand this just destroys the validity of anything else you post on this subject. It may also explain why you had so many problems with the G5RV.  Cry
Logged
KX5JT
Contributing
Member
*
Offline Offline

Posts: 1919


John-O-Phonic


« Reply #31 on: December 28, 2009, 03:47:53 AM »

Steve HUZ said:

Hey, a full-sized dipole will suck when it's only 10-20 feet off the ground and used on 80 meters.   

BUT I have worked a couple stations in FL (HOA hell) that used 80 meter dipoles at 12 feet and another at 18 feet just a few weeks ago and lo and behold!!!  20 db over 9 signals at the 100 watt carrier level!! 
Logged

AMI#1684
N2DTS
Contributing
Member
*
Offline Offline

Posts: 1967


« Reply #32 on: December 28, 2009, 09:19:13 AM »

That may be something they dont tell you about fan dipoles, while they might work very well on TX, they may pick up lots more noise, since they are resonant on all sorts of frequencies.

They do mention the harmonic radiation, but nothing about them picking up more noise.

I have the rump of the dxlb plus (160 to 10) as a 40 and up fan dipole, and noise on 40 seems much higher with it than the 40 only dipole.

I have had a lot of noise that sounds like a gasoline engine spark ignition noise, which is not there at all on the 80 meter dipole.
Its not a motor though...

I should just put up a 40 meter dipole in its place, and find something else for 20 and up.

My G5RV was up about 50 feet, and the TX seemed ok on all bands, but the rx picked up a lot of noise compaired to the resonant dipole.

Maybe that is the same effect as the fan dipole, its sort of resonant at various frequencies...

If you seperate the RX and TX performance, I think a number of antenna's work ok on getting rf into the air, but it seems like the rx performance varies widely between antenna types.

I noticed this dramatic difference when I had the dx-lb plus up (before it melted), it WAS resonant on 160, even though it was very short on 160, and I NEVER heard so many strong stapping signals on 160 before or since, tuning up other antenna's with a tuner did not come close at all.

Its quite an amazing difference between a resonant antenna and anything else I have tried, and if you extrapolate out the idea, it seems like a single band antenna that is resonant works best for RX.
It seems like it can be somewhat shortened, but if its resonant, it works well on RX.
As you add other bands, the noise goes up.

Brett

 

Brett
Logged
KM1H
Contributing
Member
*
Offline Offline

Posts: 3523



« Reply #33 on: December 28, 2009, 10:38:33 AM »

When I moved back to NH from Chitown in 73 I put up a HB G5RV as the quickest way to get on the air while working on towers and better antennas.

It was roughly 60' high and strung between a pair of pine trees. The open wire feed was roughly 400 Ohms made from #14 and the balun was HB. Running around 600W I had no problem working DXCC on 80M CW over about 6 months, helped of course with a couple of Beverages as well as contests.

It also worked well on 40 & 20 but of course not on 15 where it isnt supposed to. And 10 was dead due to the sunspot cycle.

Quote
I used to run a home made g5rv, compaired to a resonant dipole fed with coax, it was a poor and noisy performer on 80.

That is a pretty good indicator of feedline radiation.

Carl
KM1H
Logged
W2VW
Contributing
Member
*
Offline Offline

Posts: 3490


WWW
« Reply #34 on: December 28, 2009, 10:39:14 AM »



So what if it's -73 dBi down in the woodwork in that model?


You posted that info in the first place to prove something about vertical radiation.

Logged
W2VW
Contributing
Member
*
Offline Offline

Posts: 3490


WWW
« Reply #35 on: December 28, 2009, 10:42:37 AM »

out the idea, it seems like a single band antenna that is resonant works best for RX.
It seems like it can be somewhat shortened, but if its resonant, it works well on RX.
As you add other bands, the noise goes up.

Brett


According to this:

http://www.its.bldrdoc.gov/fs-1037/dir-031/_4576.htm

my 124' doublet is resonant somewhere in the 40 meter band.

Logged
W2PHL
Contributing
Member
*
Offline Offline

Posts: 112


Phil


WWW
« Reply #36 on: December 28, 2009, 10:53:58 AM »

That may be something they dont tell you about fan dipoles, while they might work very well on TX, they may pick up lots more noise, since they are resonant on all sorts of frequencies.

 Brett , I also noticed an increase in received noise when i added a second element to my 33ft veritcal. The second vertical element (connected at the feedpoint like a fan dipole) gives me 20m but the tradeoff seems to be more RX noise. Thanks for posting your findings.

Phil  
Logged

I think therefore AM
K1JJ
Contributing
Member
*
Offline Offline

Posts: 7236


"Let's go sailing, Tommy!" - Yaz


« Reply #37 on: December 28, 2009, 12:01:28 PM »

Some day I'm gonna stretch a well-made G5RV between two towers at 130' high - and work the west coast on 75M AM using 10w - just to prove a point... Grin

T
Logged

“Remember, remember, this is now, and now, and now. Live it, feel it, cling to it. I want to become acutely aware of all I’ve taken for granted.”    Sylvia Plath
KF1Z
Member

Offline Offline

Posts: 1781


Are FETs supposed to glow like that?


« Reply #38 on: December 28, 2009, 12:15:50 PM »


The G5RV is, was meant to be, designed as,  a 20 meter antenna, albeit 3/2 waves long.

What happens if you use a 20 meter dipole on 75 meters?
Logged

K3ZS
Member

Offline Offline

Posts: 1019



« Reply #39 on: December 28, 2009, 12:57:05 PM »

A 3/2 length antenna fed in the center with coax should be about the same as 1/2 lambda dipole.
Logged
KB2WIG
Contributing
Member
*
Offline Offline

Posts: 3699



« Reply #40 on: December 28, 2009, 01:02:26 PM »

I worked a w4 on 144.2 with my J pole. This proves that its OK for 2m DX.


Just a little bit more gasoline for the campfire.... ..


klc
Logged

What? Me worry?
K3ZS
Member

Offline Offline

Posts: 1019



« Reply #41 on: December 29, 2009, 10:48:24 AM »

A half-wave dipole is around 70 ohms (center fed) only when it is a half-wavelength in height over virtual ground.   At most heights normally used on 80 and 40 meters, it would be closer to 50 ohms.   It really will only result in a 1.5:1 mismatch no matter what coax you use.     Check out any of the antenna handbooks.   They usually have a graph of characteristic impedance vs. height above RF ground.
Logged
KX5JT
Contributing
Member
*
Offline Offline

Posts: 1919


John-O-Phonic


« Reply #42 on: December 29, 2009, 05:30:34 PM »

oops, we have a split thread.. I'll post this on the new thread... (i guess  Roll Eyes )
Logged

AMI#1684
W2DU
Contributing
Member
*
Offline Offline

Posts: 490

Walt, at 90, Now 92 and licensed 78 years


WWW
« Reply #43 on: December 29, 2009, 07:10:24 PM »

Hi All,

I’ve been too ill for the past few weeks to participate in the AMforum, but son Bill, W2WM, told me about the new thread discussing the G5RV. Before I became too ill to participate I started a detailed review of the G5RV, including a mathematical analysis of the so-called ‘matching section’. I intend to finish the review and post it here. I mentioned the review to Bruce (UJR), and he said he’d be interested in seeing it.

So now, if I may, I’d like to present a downright dirty abstract that will introduce the review when it’s finished. To begin, the G5RV is a good antenna, except that the design is atrocious. It radiates like any 102’ dipole, in spite of the outrageous feedline arrangement that Varney called ‘special’. I’ll probably incur a lot of antipathy with some here on the Forum with some of the remarks, which I know won’t go over well with everyone, but I have a thick skin.

First, the 33’ matching section performs no effective matching to a coax. The mathematical analysis I made that will be included in the review proves it. As Varney used it on 20m it performs no useful function, because 33’ is a half-wave on 20m—a 1:1 impedance transformer. If he had connected the coax directly to the radiator instead of the 1/2wl open wire the result would have been exactly the same. On all the other bands the 33’ open-wire line presents horrible mismatches, with high SWR to either 50- or 75-ohm coax. If you want the open-wire portion to present an input impedance near the impedance of the coax, the length of the open-wire needs to be engineered separately for each band—one length will NOT provide a match on all bands. Consequently, it is not a good arrangement for all-band use, because the feedline has too many deficiencies. For a good more simple all-band antenna that will perform every bit as well as the G5RV is a dipole around 125’ or 130’ fed in the center with open-wire line all the way to the tuna. (Varney used 102’ for the radiator only because he wanted a four-lobe radiation pattern and a low terminal resistance at resonance.) The characteristic impedance of the open wire is unimportant. IMHO, the G5RV is vastly over-rated, because it’s not simple to build for what one gets as the result of hard work.

Second, feedline radiation. If no balun is used between the open wire and the coax, feedline radiation from the coax is guaranteed, big time. Contrary to some of the present thinking, if current on the open-wire portion is equal and opposite on each side, there will be NO significant radiation from the open-wire portion, regardless of the SWR appearing on it. (The reason is explained in detail in Chapter 20 of Reflections, available on my web page at www.w2du.com.)

Third, if coax is to be used, it should be used only on 20m because of the high SWR on all the other bands. On 20m the open-wire section is of no use--unwanted baggage.

I’ll post the final review as soon as I’m able to finish it.

Walt


Logged

W2DU, ex W8KHK, W4GWZ, W8VJR, W2FCY, PJ7DU. Son Rick now W8KHK.
WA1GFZ
Member

Offline Offline

Posts: 11154



« Reply #44 on: December 29, 2009, 09:47:52 PM »

Thank You Walt and hope you feel better. Maybe some people will let this stupid antenna configuration die as it should.
Logged
Steve - WB3HUZ
Guest
« Reply #45 on: December 29, 2009, 10:37:58 PM »

I look forward to your analysis Walt. My quick analysis of the G5RV on 80 meters shows only about 1 dB of loss (not counting the balun) when using 50 feet of RG8 foam type coax. Some additional loss will occur in the tuner, but with a good tuner this should be minimal. Due to the low Z involved, a less than "good" tuner may incur larger losses.

For a G5RV at 60 feet above ground with average conductivity.

f: 3.8 MHz

Feedpoint Z: 47 - j322

Z at the end of 33 feet of 450 Ohm ladder line: 36.91 + j98.69
The ladderline loss is 0.5 dB

Z at the end of 50 feet of RG-8 foam-type coax:  9.65 - j0.78
The coax loss is 0.27 dB.
Logged
WA1GFZ
Member

Offline Offline

Posts: 11154



« Reply #46 on: December 29, 2009, 10:42:20 PM »

now what happens on 40
Logged
W2DU
Contributing
Member
*
Offline Offline

Posts: 490

Walt, at 90, Now 92 and licensed 78 years


WWW
« Reply #47 on: December 31, 2009, 09:20:54 PM »

Hello Steve,

Re your post 58 above, I'll review it carefully when I'm feeling better, and up to it. What caught my eye, however, is the 0.5 dB attenuation of the ladder line--seems a little high. Are you sure you don't mean 0.05 dB?

Walt
Logged

W2DU, ex W8KHK, W4GWZ, W8VJR, W2FCY, PJ7DU. Son Rick now W8KHK.
Steve - WB3HUZ
Guest
« Reply #48 on: January 02, 2010, 08:16:07 PM »

Does seem a little high. That what I got when I ran the numbers through the TLA program. The 450 Ohm ladder line is probably the 18 gauge type. The SWR is almost 14. I'll have to look at the graphs in the handbook and see what they say.
Logged
W2WDX
Guest
« Reply #49 on: January 10, 2010, 05:02:17 PM »

OH YEAH!!! Well ... I run an OCF with no tuner and a perfect match on 75m, 40m, 20m, 10m & 6m! It is the greatest antenna ever made and I am convinced it blows away anything you guys are talking about. That's what I've been told. YEAH!!! So there!!!!

Touchy feely ... what ... Who me?!

All the power in the world and I'm still PW!

God ... get me some real estate and some height and a nice dipole tuned for resonance and I might be able to strap W2VW for a change. Look out Dave.

John
Logged
Pages: 1 [2] 3   Go Up
  Print  
 
Jump to:  

AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
 AMfone © 2001-2015
Powered by SMF 1.1.20 | SMF © 2013, Simple Machines
Page created in 0.098 seconds with 18 queries.