Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: [1] 2 Next   Go Down
  Print  
Author Topic: Help improving my SWR on a Windom antenna  (Read 4791 times)
N3CSA
Member

Posts: 25




Ignore
« on: January 23, 2011, 04:14:31 PM »

I am using a homemade Windom.  I would like to be able to dispense with my antenna tuner so I minimize loss of signal strength.

I'm using a Yaesu FT-102 with tube finals, so SWR isn't so much of an issue as it is with solid-state rigs.  But, I would like to be able to get it down some on the bands where it exceeds 3:1.

The antenna is below 3:1 on the lower portion of 80 meters, all of 40 meters, and all of 20 meters.  On 160, the upper portion of 80, and 17 meters the SWR is over 4:1 so I can't drive the antenna directly.

So, I am curious if I can take some steps to reduce the SWR on the bands where it is currently too high.  The feedbpoint is up about 20 feet and the ends are about 15 feet off the ground (no tall trees, unfortunately).  The center has a 4:1 balun and I come off that with about 10' of coax into a ICE surge suppressor.  Beyond that, I run inside the house to an antenna tuner (MFJ-993), then to a coax switch and up to the rig.

Until it warms up outside I can't do much about the height.  But if there is anything that anyone can recommend to reduce the SWR and allow me to remove the tuner completely (I am not sure if I lose any signal through the tuner when in bypass) I'd appreciate it.

73
N3CSA
Logged
WX7G
Member

Posts: 5977




Ignore
« Reply #1 on: January 23, 2011, 05:23:27 PM »

I think you're doing pretty good for a modern "Windom" and the tuner will be your constant companion.
Logged
WB6BYU
Member

Posts: 13152




Ignore
« Reply #2 on: January 23, 2011, 06:02:54 PM »

There are various changes in the leg lengths to optimize the SWR for specific bands (or
frequencies within a band), but no combination that will match without a tuner on all
bands.  That's just the nature of best.  They do get close enough on several bands
so that losses in the feedline are low.  Generally when that is the case, the impedance
presented to the tuner can't be too extreme, so the tuner losses should be low as well.
(It probably won't have a low SWR on 15m, however.)

Actually a friend of mine tried out a design with a coil in it that was supposed to give
a better match - I've inquired to see how well it actually works.


One of the problems is that the Off-Center Fed Dipoles (commonly called a "Windom",
but that had a single-wire feed system) is common-mode current on the coax.  This
will affect the resonant frequencies and the SWR, and is impossible to predict because
it depends on the length of the coax and what is connected to it.


If you want a low SWR, your best bet is a set of parallel dipoles so you can adjust each
band individually.  If you are a CW operator you can get a horizontal 80m loop to resonate
with a reasonable match on many bands.
Logged
G3TXQ
Member

Posts: 1510




Ignore
« Reply #3 on: January 24, 2011, 08:36:54 AM »

If you are intent on achieving a low VSWR on that antenna on as many bands as possible, take a look at some of the work done by ON4AA:
http://hamwaves.com/cl-ocfd/index.html

That said, I'm not really sure why you are concerned about losses in your tuner?

Steve G3TXQ
Logged
WB6BYU
Member

Posts: 13152




Ignore
« Reply #4 on: January 24, 2011, 09:37:48 AM »

The ON4AA material is an excellent reference - I suspect that is the design that my friend is using.

Do notice, however, that the 80m band is Europe is only 3.5 - 3.8 MHz, so the SWR will be
somewhat higher at the top of the SSB segment in the US (up to 4 MHz.)
Logged
KG6YV
Member

Posts: 508




Ignore
« Reply #5 on: January 24, 2011, 10:03:37 AM »

I suggest you join the Yahoo "windom antenna" group and get some very good advice from hams there.

Greg
Logged
N3CSA
Member

Posts: 25




Ignore
« Reply #6 on: January 24, 2011, 05:19:39 PM »

Thanks for the suggestions. I didn't know there was a Yahoo group for the window, cool I will check that out.

As far as a tuner, I don't want to have to use the tuner and lose signal strength. 
Logged
K0OD
Member

Posts: 2546




Ignore
« Reply #7 on: January 24, 2011, 06:25:14 PM »

"I don't want to have to use the tuner and lose signal strength. "

You were asked to explain that odd statement. Tuner losses are generally on the order of a dB or less... a difference that can't be heard. Yes, there would be some tiny loss even if the tuner is bypassed.

Getting the windom higher when the weather improves should be your priority. If 15' to 20' is as high as you can get it, consider a ground mounted vertical especially for the lower bands.  You haven't mentioned what sort of operating you prefer... rag chewing, DX, etc, and the bands you like.
Logged
WB6BYU
Member

Posts: 13152




Ignore
« Reply #8 on: January 25, 2011, 04:30:51 AM »

Quote from: N3CSA
As far as a tuner, I don't want to have to use the tuner and lose signal strength. 


Tuners can have a lot of loss under certain circumstances, but most of the
time it is insignificant.  In many cases it is better than the alternatives.

The worst cases are for low impedance loads on the lower bands (especially 160m)
when the capacitors aren't big enough.  With an SWR of 10 : 1 (10 ohm load)
on 80m, the worst case loss is just 1dB, which is about the minimum difference
that can be detected by ear.  But 500 ohms (also an SWR of 10 : 1) has a loss
of only 0.2 dB, and the losses in your feedline (even when matched) may well
be more than that.

So as long as the SWR at your tuner is less than about 10 : 1 on 80m, the tuner
losses should be negligible.  Beyond that it will depend on the coax length as to
whether the tuner sees a a load impedance that it can match efficiently or not.
My general guideline would be to try to keep the SWR to about 5 : 1 at the tuner -
that not only minimizes the losses in the tuner, but also in the feedline.  Beyond
that there isn't much to be gained unless you have a long or lossy feedline - but
that isn't due to losses in the tuner.

There certainly are some bands where the standard Windom won't present a low
SWR, and it may be worthwhile improving the match there, especially if they
are of interest to you.  On 15m, for example, the SWR is likely to be 10 : 1 or
more at the antenna, and you can easily lose half your power in the coax,
but the loss in the tuner is far less than that (about 4% of your power.)

So don't automatically assume that tuners are lossy.  They can be under some
extreme circumstances, but more often the losses in your feedline will be
of more concern.
Logged
K1BXI
Member

Posts: 812




Ignore
« Reply #9 on: January 25, 2011, 11:45:15 AM »

Any loss of rf in a tuner is turned to heat and most of a tuner loss is in the inductor. It only take a very few watts to start heating the coil.

Does yours get hot, or even warm?...if not, as others have said, in most cases tuner loss is at a minimum.

John

Logged
K0OD
Member

Posts: 2546




Ignore
« Reply #10 on: January 25, 2011, 12:30:28 PM »

I did have a lossy tuner once. That was on 160 meters. I wasn't concerned about signal loss which was still probably small. I was mostly concerned with the smell of melting plastic and the smoke that filled the shack and indeed most of our house when I ran a KW on that band. Ultimately, the costly tuner inductor would have melted. That same tuner, into a similarly mismatched antenna, only got warm on 80 and was fine on higher bands. 

160 requires a pretty hefty tuner from my limited experience on that band.

Logged
K1BXI
Member

Posts: 812




Ignore
« Reply #11 on: January 25, 2011, 04:13:26 PM »

Sounds like one of those old BW Air Dux coils. I deformed a 3" 4 tpi # 10  in a pi net with a home brew 3-400 gg on 20 meters once. No flames or smoke, but it did have a Lucite odor about it.

Didn't look too pretty either...........I miss those days.

John
Logged
N3CSA
Member

Posts: 25




Ignore
« Reply #12 on: January 26, 2011, 04:50:51 PM »

Thanks all, but back to my original post.. I am looking for ways to improve the SWR.  Is there anything I can do (better patch cables, maybe of a specific length, or other steps I can take?  Or is it completely and totally dependent on the antenna itself?

thanks.
Logged
WB6BYU
Member

Posts: 13152




Ignore
« Reply #13 on: January 26, 2011, 05:49:26 PM »

Yes - connect an additional one hundred feet (or more) of RG-58 coax between
the antenna and the rig.  That will lower the SWR, but only because it adds loss.
(But you still won't get a good match on 160m if the antenna is cut for 80m -
the impedance is just too wild.)


There are steps that you can take to reduce the SWR, but they won't necessarily
work for all bands simultaneously.  Presuming you can't do anything to change
the antenna, the first step is to determine what the actual impedance (not just
the SWR) is at some point on the coax (probably at the shack end of the coax.)
Once you know that you can add fixed or variable capacitors or coils, or stubs
of coax, to improve the match.  But this is, in effect, simply an alternate form
of an antenna tuner, and probably won't be any more efficient.

Sometimes, though, a standard tuner isn't as convenient as we'd like, even after
marking the settings for the various bands on the tuner face (I use pieces of
yellow stickies.)  I like the convenience of band switching without having to
retune, so  I built a switched tuner based on a big multi-section switch.
Most impedances can be matched with some form of L-network, and I usually
use one switch section to choose an input capacitor (if used), one for a
coil tap, and one for an output capacitor.  The capacitors are fixed micas
from the 1950s, which are generally adequate for 100 watts unless the
impedance is high.  In some cases I use trimmer or variable capacitors.
But you can use any sort of circuit that you want - you just need to know
the impedance that you are trying to match, or have some way to experiment
to find what components you need to match it.  I generally start with a
variable capacitor and a coil that I can tap, and use those to determine the
values I need in the tuner.  It takes some work, and has to be repeated when
I change the antenna around, but once I get it set up I can change bands
by just turning the switch.

It's just an antenna tuner, of course, but more convenient in some cases.
Logged
N4JTE
Member

Posts: 1155




Ignore
« Reply #14 on: January 26, 2011, 06:00:10 PM »

You are trying to make an all band antenna from a very low windom, i know you are restricted in height so i would suggest using 450 ladderline. If you take the time to tap into the approximate 1/4wl point on the line and short the end you will find a sweet spot for each band. Time consuming and a lot of ladderline if 160 part of the plan. I use a 40 meter edz, 185ft at 50 ohms and on 80 same antenna with another tap in about 5 ft away also gives me 50 ohms direct.
The use of a tuner/loss, is miniscule if the antenna is 1/2 wl on lowest band of interest.
Regards,
Bob
Logged
Pages: [1] 2 Next   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!