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Foxhunter
Member
Username: Foxhunter
Post Number: 79
Registered: 4-2008
| | Posted on Monday, August 04, 2008 - 3:08 pm: | 
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It's me asking more questions about SWR on a base-station antenna. It's the same one I posted about with other questions, a Shakespeare Super Big Stick model# 76SU.
My SWR gets lower the higher I go in channels on 11M. It's 1.5 on the lower channels and goes gradually down to 1.15 on the uppers. It is a nice (not really!) "even climb" of SWR the lower I go in channels.
To start I worry because mainly of amplification and that tends to increase SWR even a little more to begin with. So that 1.5 SWR may be a 1.7 or so with a 480W amp on which is something I don't like.
The antenna isn't tuneable in any way that I'm aware of. So it is an 11-M antenna and should be pre-set for resonance in that band-range. If it were a mobile whip I could easily raise the whip to even-out the SWR and tune it more evenly so that SWR is centered on CH.20.
Any ideas on what to do to change/improve the SWR on this type of antenna without tuning rings? Is my only answer the use of an antenna tuner/matcher? That would improve "displayed readings" with what the radio "sees" but wouldn't solve the problem. Anybody? I'd appreciate the advice. Or am I beating a dead horse and tiring you guys out with this antenna? |
Patzerozero
Senior Member
Username: Patzerozero
Post Number: 4357
Registered: 7-2004
| | Posted on Monday, August 04, 2008 - 7:44 pm: |
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how high above the roof/trees/etc is it? try raising it and/or moving it away from ANYTHING within about 18 feet of it.... |
Tech565
Moderator
Username: Tech565
Post Number: 10
Registered: 7-2008
| | Posted on Monday, August 04, 2008 - 7:59 pm: |
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Also make sure your jumpers are good. Sometimes just changing a jumper can make a big difference. But, I do agree with Pat. Try moving the antenna in all directions to see if there is any improvement. But to have a 1.7-1 SWR with an amp is still in the acceptable range. I also hate to say it, but, you might have to change antennas as that one might have issues.
Tech565/Ham243
Net and Asst. Net controller
CVC#38
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Tech291
Moderator
Username: Tech291
Post Number: 514
Registered: 11-2004
| | Posted on Monday, August 04, 2008 - 8:14 pm: |
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Foxhunter,
1.7:1 wouldnt bother me at close to 500 watts.When your swr drops at higher freq's it is showing to be more resonant at higher frequencies,or shorter than necessary for midband(around 27.195 mhz) on 11 meter.One method to bring it down on that type of antenna is maybe put a couple stanless or brass washers on the threaded stud of the whip.Bear in mind it may only drop the center freq about 2 channels.It looks like your antenna may have been trimmed for 10 meter use,didn't you say once before it was used formerly by an amature operator?
Tech291
cef291/cvc6
kc8zpj |
Foxhunter
Member
Username: Foxhunter
Post Number: 80
Registered: 4-2008
| | Posted on Monday, August 04, 2008 - 8:44 pm: |
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Well the antenna is 16-FT tall to begin with. It is mounted above my roof-line of my 2-story house.
The antenna's base begins at 25-FT above ground then is 16-FT tall totalling 41-FT to the tip. It is free & clear except for a nearby treetop that is within 15-FT of the antenna on one side. It is mounted on/above a large existing TV antenna and I am currently limited to this location until a tower comes my way (I am working on it).
I am just so used to trimming or lengthening whips to tune a mobile antenna and am not sure why a "pre-tuned" antenna such as this would display slightly out-of-tune characteristics like this.
Jumpers are definitely good and I have switched them for identical lengths, but I usually won't exchange them for different lengths to alter my readings.
Yes that's correct the antenna was used by an amateur operator who abandoned his antennas at a house other occupants are now residing at. Hmmm. |
Marconi
Advanced Member
Username: Marconi
Post Number: 786
Registered: 11-2001
| | Posted on Monday, August 04, 2008 - 10:07 pm: |
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Foxhunter, I suspect this antenna is a vertical dipole that looks like an end fed 1/2 wave. If I'm right, the ends are very sensitive to objects being nearby. Sounds like the bottom end may be close to the other antenna it is mounted above. That might be a problem that could effect the match. |
Foxhunter
Member
Username: Foxhunter
Post Number: 82
Registered: 4-2008
| | Posted on Tuesday, August 05, 2008 - 10:17 am: |
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Well Marconi I used this mounting locating for two reasons. It was already high up and had clear space surrounding it-----the large Yagi-Uda TV antenna is at 25-FT above ground and already mounted there as the best suitable location on/attached to the house.
Also mounting above the TV antenna I thought would be additionally beneficial giving additional reflect with a large amount (32) of radials below the antenna. The Big Stick's makers over at Shakespeare tell me the antenna was designed with a "built-in ground plane---no radials needed" is what they wrote.
I "assumed" (you know what that often can mean) that "adding" radials below (in a sense by mounting above the antenna) would not "harm", but improve reflect perhaps, although the radials nearby are not tuned-for-length.
The bottom 12-inches of the Big Stick is actually beside the TV antenna radials because I have the Big Stick clamped to the TV antenna mast. Maybe there is a slight negative interaction of some kind between the two antennas? I will at some point try and mount the antenna even higher above the TV one to see if this changes anything. I'd appreciate any followup comments from you guys. This must seem like real novice childs-play compared to what some of you guys do/have done. |
Hotwire
Senior Member
Username: Hotwire
Post Number: 2562
Registered: 1-2005
| | Posted on Tuesday, August 05, 2008 - 1:35 pm: |
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Are you using an external SWR meter or the radios built in SWR meter? Also, are you resetting the SWR meter each time you change channels?
KEEP IT REAL!
Kenny
cef491(27.115lsb)
2sf491(27.555usb)
Indiana
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Foxhunter
Member
Username: Foxhunter
Post Number: 83
Registered: 4-2008
| | Posted on Wednesday, August 06, 2008 - 2:51 am: |
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Thanks yes I am using an SWR bridge in-line at the end of the chain, and not the radio's own.
I always check the SWR at Channels 1-20-40 separately, recalibrating each and every time. This is the first thing I do each time I sit at the station (and several times throughout to be sure).
After being reasured that all is satisfactory I go on to check the same channels again with the amplifier powered up, to see again if I notice anything unusual. I believe in instruments, and faithfully use them in all things.
I've always tried to tune my mobile antennas for center around CH.20 to have it centered right in the middle of the band there (because I talk on all channels). So with a base-antenna I expected to see an SWR fairly even across the channels at about the same reading whatever it might be---not to "taper" up or down.
I thought a base antenna at proper length would display a fairly even SWR as it is already (supposed to be) tuned for 11-M.
From/at CH.20-----if I go up in channels from there the SWR keeps gettting lower, all the way to CH.40
From/at Ch.20-----if I go down in channels from there the SWR keeps climbing/rising, all the way to CH.1
I know that's not an alarmingly high SWR----but it does bother me knowing something isn't right. And of course if I turn the amp on, SWR gets a little higher even still----and that's when/why I worry about what is going on.
Talking (for instance) on CH.6 around here is big and I am not comfortable really with a 1.7 SWR there, but adding an amp and getting into the SWR 2's means I may be harming my amp and also that I am wasting power that's being re-directed back down at my equipment, so it takes "the fun" out of it for me because of that, knowing there's a problem. |
Drzuo
Junior Member
Username: Drzuo
Post Number: 32
Registered: 1-2006
| | Posted on Wednesday, August 06, 2008 - 6:48 am: |
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You may be getting reflection from near by objects, or interaction with some of the other antennas. Best practice(but not always possible) is to try to get the antenna 36 feet from other antennas. 1.7:1 isn't that bad and power loss is minimal. I know some amps are touchy, but you should be fine anyways. Another thing I have seen, antennas will tune a bit by raising and lowering the mount on the mast its mounted on. If it uses a dual "U" joint type mounting mechanism with the threaded U shaped clamps try raising or lowering where the mount sits on the pole. Good Luck. |
Sonny
Intermediate Member
Username: Sonny
Post Number: 181
Registered: 5-2004
| | Posted on Wednesday, August 06, 2008 - 8:02 pm: |
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Fox, some attenas have a swr curve and will be average 1.1 in the middle and then either go up or down as you get further away from center. I have a 4 element beam on a 30 ft. tower and I tried ajusting the gamas in and out but could only aquire a 1.5 swr. I use a MFJ 962D tuner now.
Nobody on the other end will beable to tell if you have a 1.1 or 2.0 swr. Also to see one "S" unit on a receving end you would have to go from say 4 watts to 600 watts. Hope to hear you sometime here in Delaware |
Foxhunter
Member
Username: Foxhunter
Post Number: 84
Registered: 4-2008
| | Posted on Wednesday, August 06, 2008 - 8:16 pm: |
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It is clamped with large "radiator" type clamps to the side of the TV antenna mast as it was already there and gave me good height and mounting location above the house. I have just got some extra masting and will try mounting it higher above the TV antenna. Having it above the TV antenna seemed like a good idea because of all those radials going to waste.
It is possible and I have thought that the two of you suggesting the former owner & Ham may have altered the antenna somehow. It doesn't look as thought the antenna can be taken apart to do so though the "innards" are encased in a fiberglass shaft. |
Foxhunter
Member
Username: Foxhunter
Post Number: 85
Registered: 4-2008
| | Posted on Wednesday, August 06, 2008 - 8:22 pm: |
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Sorry about the double-post but this one from Sonny just appeared. Yes I've thought about what you're saying about an "SWR Curve" and that could make sense. However mine is not centered and I would prefer it to be for sure.
The lower channels have a high SWR and the upper channels have a low SWR. As I go from channel 1 on up the SWR gradually is dropping, to its lowest reading on channel 40.
I certainly "get out" to Delaware, often I am in the very center of South Jersey. What channels do you hang out on? |
Sonny
Intermediate Member
Username: Sonny
Post Number: 182
Registered: 5-2004
| | Posted on Thursday, August 07, 2008 - 4:24 pm: |
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Fox, click on my name and you will see where I hang out on |
Twowatt
Member
Username: Twowatt
Post Number: 80
Registered: 7-2005
| | Posted on Tuesday, September 09, 2008 - 10:50 pm: |
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Fox,
I don't recall ever seeing the 16' Big Stick, however, if the top section has a 3/8" x 24 threaded shaft on the bottom of it, replace the top section with a 7' Skipshooter -- it's adjustable. If you want a more broad-banded antenna, try to find a 5' Quad Wound (used to be made by Van Ordt) (I have had both of these antennas; and they covered 26-28Mhz easily and I talked 30-40 miles normally, with only a Mirage 88)
Or use the Wilson trucker 10" + 49", or maybe even the K40+. Both are adjustable.
regards |
