The AM Forum

Blew the 75 meter traps.  40 still works

Al

Too much power or water and weather? I thought traps seldom fail, last decades.

Why use traps in the first place? If 40 and 80 is all you want/need, a fan dipole will work FB with no power or moisture related crap outs.

If you don't have replacement traps, can you easily lower the antenna and at least for now replace the traps with insulators and then use removable jumpers to shift from band to band?  Not automatic, but at least you will be back OTA.  I don't have traps, but I use links on the end of  my 75 meter inverted vee to add about 3 feet to each side  to resonate it down on the CW end of the band.  I can drop the ends of the antenna to connect and disconnect the extensions.  I am currently using small electrical panel ground /neutral bar connectors as the link connectors.  They are small aluminum bar stock drilled with 5 holes and set screws, just connect a jumper from the extension pieces to the main legs. Stocked in the electrical connecter section at the Depot near the wire nuts and lugs.

The gurus who repeat that a lot seem to have some $ interest in traps.

Ain't a trap in North America can bother me.

I have no traps and do not want any. I see them as a compromise that brings benefits along with a reliability risk, but I thought the risk was small.

It's not that traps are inherently bad. But they are another component in the system that can fail. They are another component in the system with a certain amount of loss (it can be very low if properly designed and built but the trade offs are further complication, cost and weight). If you want multiple bands with one wire, traps can make that possible. In this case, for just two bands (80 and 40 meters), I see traps as an unnecessary complication given that a fan dipole is easily created.

Al,
Why use traps?  Bandwidth of antenna is reduced and its just another place to add loss.  I think you have the real estate to put up a full size antenna.  Go with open wire line and tune it if you want multi band operation. 

If that won't work for you, then Steve's suggestion of a multi-band dipole is a good way to go if coax is your thing.

Joe, GMS 

Traps have an impossible job on the higher band when operating out of resonance QRO. Move 75 kc and see stuff dripping out of the sky onto your new BMW's hood.

That alone is enough to look elsewhere for another way to make the skyhook take energy.

Then there's weight, ugliness and weather.

I ran an 80/40 W9INN for years. He didn't call them traps. He called them resonactors. Nice word, don't you think? It fit the space perfectly. When I switched to full length antennas, no one could tell the difference. Admittedly I didn't do A/B tests but at the time I was in daily, lengthy QSOs with people all over the state. These guys noticed everything and did not notice an iota of change.

Used the set up below for 3-4 years. Wire was insulated #12 stranded (Home Depot/Lowes stuff). Height was about 70 feet. All FB.

(http://www.amwindow.org/misc/newant/FandipWorksheet.gif)

(http://www.amwindow.org/misc/newant/40End.jpg)

(http://www.amwindow.org/misc/newant/Center.jpg)

(http://www.amwindow.org/misc/newant/WireSky.jpg)

Hi Jon,
I do believe if you ran an A/B test and all other things being equal which is sometimes hard to do that the full size antenna would definitely have an advantage.  Just do an engineering loss comparison on the two designs.  Its just like guys with G5RV saying they work as well as full size antennas on the lowest frequency of operation.  Plot an SWR curve on a trap antenna and you will see that the bandwidth is narrower.  Then some ops use and antenna tuner and think that helps the radiation efficiency model !!   

I have nothing against traps or G5RV and I realize that some folks have little choice so they use them.  

Joe, GMS    

I have used a similar antenna years ago and it worked very well.  If I ever get sick of my open wire line that is what I would switch back to. 

Joe, GMS

I later put up a reflector for 40 meters to create a 2 element Yagi pointed west. Ran out of time to put up a reflector for 80. It probably would not have worked all that well anyway, given the height.


(http://www.amwindow.org/misc/newant/UpCoax.jpg)

(http://www.amwindow.org/misc/jpg/2elhuz.jpg)

(http://www.amwindow.org/misc/jpg/2elhuz2.jpg)

I also use the fan dipole method here for 160 and 75 meters.  The only difference is that the antennas are at 90 degree angles to each other.

I'm going to try the arrangement Steve HX uses when I return to Rattlesnake Island after ice out.  I need to be able to get on 160 meters from up there.  Open wire line is not an option at the Island.

If I had to do it again, I'd use wider spacing - more like 4-6 inches or make the 40 meter legs very taut. The wind once flipped the 40 meter leg around and the two wires made contact. There was some zorching and arcing. It burned through the insulation and also crapped out the SO-239.


(http://www.amwindow.org/misc/jpg/zorch.jpg)

(http://www.amwindow.org/misc/jpg/zorch1.jpg)

Steve. Does the length of the spreaders matter? Have lots of 4 and 6" plastic ones hanging around.

OK, nm. I didnt see that last post from you a few minutes ago.  ;D

No problem. I was in the middle of answering anyway.  ;D

It matters but not very much.  It just means whatever amount of interaction there is between 80 meter dipole and the 40 meter dipole is different. Either way, you will likely need to trim one or both of the dipoles, so it's really irrelevant. You can use the speaders you have and like I said previously, I recommmend a wider spacing.

I haven't had to trim a wire antenna in years.

Not physically.

Ive run 160/80 common feed dipoles for decades and the current config is 160/80/75 so rigs/amps that cant handle the VSWR work well. I dont bother with tuners or open wire, coax is all I'll use.

The 80/75 are seperated by about 30 degrees and a bit off 90 degrees from the 160 because of limited tie off points in the woods unless I want to do a lot of clear cutting. A little wire trimming brought everything where I wanted.

At 180' apex its a bit too high for photos. Many times Timmy tells me Im the only station he can copy 100% when on his mobile runs and thats just with various exciters.

With the 40M yagis down for changes Ive added a 40M inverted vee at right angles to the center of a 80M sloper which starts at 100' on another tower. That
required no tweaking.

The fan dipole is certainly an attractive option.  The reason I stayed with the trap approach is that after almost 30 years of flawless performance, I retired my trap 80 / 40 meter dipole only because the wire at the center of the dipole was about to part way.  Adding 160 meters was a natural progression for the time being.  Unfortunately, I'm quite sure I zorched the 75 meter trap.

I will discuss the fan option with my friend Bob, KB1QV who is my antenna monkey.  He is the one who climbs up and down my tower, house roof etc.  The trap dipole was premade and ready to put in place.  I should have figured out that something was fishy because I had an unusual crap out of the SDR program two successive nights before the big crap out of the antenna.  I think I was arcing over on one of the traps and that was what was the culprit for the program hangup.  The plan is to lower the antenna just enough to replace 75 meter traps with insulators and will just have to be off for the winter on 160 (sigh).  I will probably have to readjust the length of the 75 meter portion with the removal of the 75 meter traps.

Even though I have dropped my power by more than 10 dB and am operating into a really screwed up VSWR I still get out pretty good but I really need to not stress the automatic tuner in the Flex.

I'm pretty sure that Joe WA2PJP is using a fan dipole and he is quite happy with it so between Steve's and Joe's and others on this thread recommendation, I will certainly discuss this option with Bob.

I'm still pretty good on 40 meters so I may see you all on heavy iron night on 40 - the freq is 7160, right?

73, es Happy New Year all

PS:  I can't say enough by way of appreciation for the help that Bob has given me in the past few years with my antenna.  He is also the one who kept badgering me about tearing down the old garage and putting up the new shack.  Without his encouragement, I would have a silent key before my time

Gee I am sorry I burned out your antennna Al.

Mike

Mike, I am not sure if it was you or me..   Al, I drove past your QTH on my way down Sinclair Ave. to the cell site Friday morning.. Maybe my 12 watts of AM  was too much for your system..  ;D

Well, I'm back in the saddle again.  Bob QV came down - we traipsed around in the snow.  Got the new dipole up just a little after dark.

Steve TAV - good talking to you today.  Too bad you couldn't have stopped in and visited the shack.  At least you know what it looks like from the outside.  One good thing came out of this: I now can raise and lower all three sky hooks.  Shouldn't have to climb up the main roof for the foreseeable future.

Al

Was it Karl/Chicken Neck who lit his traps up when he tried out his new Class E rig a few years back? I seem to recall something about 'the antenna is on fire'.

Traps are a compromise at best, and as folks have said Al, just another point for problems. Keep it simple and you'll be happier long term with the results.

Well, I'm going to look into a fan dipole so if anyone has dimensions for an ant that is 70 off the ground at the apex, I'd be interested.  Probably not going to happen until spring or at least the snow is gone.

The bands of interest are 160, 75 and 40

Al

Hey, Al! You are a prophet. On 12 January 2009 you posted the following.



Actual post:  http://amfone.net/Amforum/index.php?topic=18246.msg127335#msg127335

We always new Al had special qualities!!

Joe, GMS

I ran a fan dipole for 20, 40 and 75 for a year and when the wind blew the loading would change significantly.  I have gone to an off center fed dipole using a 4:1 balun and it works great on all the bands from 75 thru 10.  I do use a Millen antenna tuner to touch it up on each band.  I have really good success with the OCF antenna.  Just my .02

Which OCF design? I'm pondering a second run at an OCF simply because the trees are in the right places. Maybe this time I can do a better job keeping the feed line radiation down and the attendant RFI issues controlled. That's what made me take down the one I used to have. I was talking through the stereo, blinking the lights, pushing the cable modem off line, etc. Still those trees are calling me.

Not to argue but antenna endorsements with no details are not that meaningful.   How high is your antenna?  How much power do you run?  Do you make long transmissions?  What stations have you worked (how far away)?. 

There's a classic article in QST by N6BT called (IIRC) Anything Works.   You can make QSOs with a lightbulb for an antenna if you run enough power and it's a big enough light bulb.  But it isn't a very good antenna.  However the op with the lightbulb might think he's had really good success with it.

There actually has been a great deal of analysis and testing of OCF's ranging from the late, great LB Cebik's articles to ON4AA's latest modeling and clever designs (http://HAMwaves.com/cl-ocfd/). There is even a Yahoo group with questions and answers ranging as usual from the completely uninformed to the quite expert (http://groups.yahoo.com/group/windom_antenna/). There are many designs each with something slightly different to recommend it usually in the form of different multi-band trade-offs. It's usually hard to get decent performance on 80 and 40 which, of course, are the two bands I care most about for this antenna. Debates about BALUNs rage on.

As I said, I ran one for years because that's where the supports were and may try one again for the same reason. I'm always curious what someone is actually using, the W8JI design, the ON4AA design, or a commercial one just to get another perspective.

Incidentally, having run fan dipoles, trap  dipoles, OCF's, coax fed full length dipoles, ladder line fed dipoles, in V's and flat tops, and a giant horizontal loop among other things, the only antenna I've ever gotten excited about was the phased dipoles which JJ called possibly the world's best 75M antenna. I still wax nostalgic over that one.

It behooves one to measure the feed line current to make sure they are equal when using open wire line.  If not balanced, feed line radiation will occur.  I found out I had about a 15% imbalance when comparing currents in each leg of the OWL.  The physical length was equal but the electrical differences caused the imbalance in the actual current.  Mine antenna is a center fed, 260' long Inverted Vee up 60' at the apex.  I tweaked the physical length slightly and got the currents to be identical and all the RFI issues went away.  In fact I had less EMI problems with OWL than coax.  The key is to make sure everything is balanced from an electrical standpoint. 

Joe, GMS     

Joe, you are the first person I've seen who has identified this problem and corrected it.

There's a lot of talk about measuring imbalance but not much talk about the cure.

Same antenna but cut to resonance (current IEEE definition) would not have allowed this measurement.

How would you feel about an electrical solution at the coupler?

Here's a good way to check your open line balance

http://www.wz5q.net/index/shack_data/tuna.htm

I like this approach

Al

Joe: Watch your quote brackets; it's hard to figure out who's saying what to whom when you embed your response inside someone else's quote.

What?

The one advantage I can see for compensating for feed line imbalance at the tuner is that it would allow you to adjust for things that are only temporarily causing imbalance, like here where in the summer I have leaves that affect one side of the dipole but aren't there in the winter.

In my case, no leaves near the antenna.   The physical sides are not electrical symmetrical due to angle and height differences in reference to the ground below the antenna.  Once I tweaked one side of the Inverted Vee to produce the same current in each leg of the feeder, the feedline radiation was eliminated.    It has remained balance for years and hence no RFI issues.  

My point is to make sure you measure your feeder current in each leg when using OWL and try to obtain the same current in each leg.  When OWL is balanced it will not radiate!  

Joe, GMS  

Interesting thoughts.

I recently have been experimenting with feeders with broken insulators and eliminating local neighborhood noise. Good housekeeping sure makes a lot of difference especially during the daytime on 75.

Things cooled off and got much quieter here after ditching coax on the low bands. To be fair I didn't try any BalUnz at the feedpoint though.

Very interesting..

Balance is problematic even with pruning of the OWL. The reason is topographic and near field influence with each side of the antenna, as most of us aren't even close to free space on the lower bands for sure.. I am using thermocouple RF ammeters inline with my OWL, and they read almost square on 160, 75 and 40. The higher bands get a little unbalanced.. Wavelength and near field structures (like power lines, houses underneath, TREES, etc) make it almost frequency specific the shorter the wavelength.. So my question Joe was the physical length of the OWL the cure all to the in shack balance?

PS. Glad Al got back on da air!

Inquiring minds..
Jeff

In my case Jeff, I made some tweaks to the length of one side of the inverted vee until the feed line current was equal.   

Joe, GMS   

Here's an example of not reading through the entire post.. Got it! You did pruning on one leg of the inverted V.. I would imagine the same result would be pruning the length of the OWL to that side as well.. I wonder if the OWL then becomes polarity specific with balance not being perfect from tuner or in the shack.. I wish I was completely balanced personally.. :)

Thanks Joe!

Personally balanced? That must have been coined by a _____. Answer determines your absolute value profile.

The only people who were ever perfectly balanced personally were Mo, Curly, Larry and sometimes Shep.

Is it possible to make one side of the OWL shorter than the other to balance the antenna?

Terry,

Mechanically that is tough to do while maintaining the line balance.   Either tweak one side of the antenna length or pick your tap points on the tuner to assure the same RF current in each feeder.  Remember the importance of the two feeders being parallel so that cancellation can occur with the opposite phase of the feeder current. 

Joe, GMS     

Ever do any close in (< 10,000 feet) signal comparison on 75 while adjusting for minimum common mode on the feeder?