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To Trap the antenna or not?




 
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WA2SQQ
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« on: November 05, 2018, 10:08:03 AM »

This weekend I discovered some age-related problems with both my 75 and 40m antennas. Rather than repair, it’s probably better that I replace. I just acquired several hundred feet of the #10 solid copper wire used by railroads.  The question is, should I feed two separate dipoles from one feed point, or make one using traps? They will be inverted V’s. I know this is a very fundamental question, but I like considering opinions and options that I may not have considered.

They will be fed with coax, with a choke at the feed point. If I cut the 75m first, just wondering how much interaction will take place when I connect the 40?
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K1JJ
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« Reply #1 on: November 05, 2018, 12:28:13 PM »

Hi Bob-

I would not fool with traps, period.  

The question is whether or not you can put up separate 40M and 75M dipoles or must feed them at a common point.  Separates are always best because you can control heights for optimum take-off angles, orientation for directivity and remove the variables and losses associated with traps.  IE, depending on your goals (local or DX, NE/SW  or N/S, etc.) they can be individually controlled.   IE, A 40M dipole at 20' is VERY different than one at 45' for local vs: DX work... and you want the 75m at least 60-90' high for local work... )  Common point feed requires you to make a choice of compromise.  Whereas, separates can be optimized in all aspects.  Antenna modeling software will clearly show you what you need to do.

If you do decide on common feedpoint dipoles, then yes, the 75M will affect the tuning on the 40M -  but the 40M will be almost invisible to the 75M, but affect it slightly.
Cut the 75M to its normal length and then expect to play around with the 40M until you get its swr where you want. A few back and forth cut and try trials with the MFJ-259B and you should get all tuning where you want.

T

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WA2SQQ
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« Reply #2 on: November 05, 2018, 01:07:18 PM »

Unfortunately, my property is limited, with zero trees. The antenna feed point will be from atop the 50 ft tower - hence the inverted V. I'm agreeing with the idea of two separate dipoles - easily done since I have 4 open slots on the antenna switch mounted at the base of the tower. The switch is fed with 7/8 hard line back to the house. Several people said, just go with a trap antenna, but I'm very reluctant because they twnd to narrow the bandwidth.
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K1JJ
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« Reply #3 on: November 05, 2018, 01:55:12 PM »

Yes, separates would be FB.  Now, decide who you want to talk with on 40M.  

On 40M I have a "local" 40M dipole at 20' high. It works BIG locally. There is a lot of local activity out 200 miles or less.   I also have a 3 el 40M wire Yagi at 60' beaming SW.   This antenna is often 15-20 dB louder out 500-1000 miles SW in the USA than the 20' dipole.   The 3 el Yagi is 15-20 dB weaker locally, just the opposite. This covers a lot of angles. The difference is due to mostly to height above ground.


On 75M, 50' is the lowest you want to go, so 50' is it for you.


Now decide who you want to chat with on 40M. My guess is 25' high will get good compromise local and farther out coverage.   Believe me, if you put the 40M antenna  at 50' you will be PW locally and strapping for DX. But that may what you want.   Though, 50' will give you some high angle due to the two lobe split.

The BEST way would be to put the 75M dipole at 50'. (max)  Then use a pulley at 50' for the 40M dipole  and pulleys at the ends and  experiment with height. Raise and lower it. You may be able to make it flat when lower rather than an inverted V, which is good. You may find that over time, following the seasonal conditions is a lot of fun to mix up your activity. Sunspots will affect performance too over time. To be able to change the height of your 40M dipole from 1/4 to 1 wavelength high would be the biggest advantage (and fun) you could have on 40M.

T
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“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

Favorite Song - Trololo thru the years:  
https://www.youtube.com/watch?v=IVqUecYGnoM
K1JJ
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« Reply #4 on: November 05, 2018, 02:34:50 PM »

Bob,

I thought of an even better idea, if you're game...

Put a 40M dipole at 50' and another one at 20'.  Run two equal length coaxes into the shack, one for each dipole.  

Tie the coax feedpoints together in the shack to get a single low angle lobe SW and NE.  Add  45' of coax to one dipole to get a "cloud burner" local lobe. (1/2 wavelength phasing line)

You could then switch in the shack for local or DX.  It doesn't get any better until you go to stacked, rotary Yagis.  You'll have so much fun with this system you'll send me a check every month... :-)

The coax phasing calc:  492/7.2 MHz  * .66 vel factor = 45' long

More precise matching (swr to 1:1) of the array can be done if you are interested, but the above example will do.

T
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“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

Favorite Song - Trololo thru the years:  
https://www.youtube.com/watch?v=IVqUecYGnoM
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« Reply #5 on: November 05, 2018, 08:49:24 PM »

Just a little note here where Tom says cut the antenna etc. I have no argument with anything he says other than to suggest using half bolts at the end insulators where you could fold the wire back on itself. I used to actually cut it at the operating point but later on when I wanted to change the resonate point, I had to add wire. Now I just fold it back and adjust it according to where I need to be to get the best SWR. I use one half bolt right at the insulator and the other at the end of the folded back wire.

I  would have liked to have experimented with traps but I think they would be a problem with resonant issues, from ice or wetness problem here in the NE?

Just my 2C worth........
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K1JJ
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« Reply #6 on: November 05, 2018, 09:05:24 PM »

suggest using half bolts at the end insulators where you could fold the wire back on itself. I used to actually cut it at the operating point but later on when I wanted to change the resonate point, I had to add wire. Now I just fold it back and adjust it according to where I need to be to get the best SWR. I use one half bolt right at the insulator and the other at the end of the folded back wire.


Yep, this is correct.  My previous post made it sound like I actually cut the wire to tune it, but no.  Folding an antenna leg wire back on itself works FB.  I usually twist it back on itself, sometimes several feet when in doubt, and it cancels out. No need to actually cut it and add back later on.  This goes for insulated wire too.

However, I've never run precise measurements with an MFJ-259B,  so cannot say for sure if there are slight, though negligible capacitive end-effects when doing it. Would be a good experiment to try sometime.

Related subject - for what it's worth:

You could say an antenna loading coil needs some leg length to work against. Just like coil placement on a mobile whip.  For example a loading coil at the center of a leg might need 30 uH to resonant the leg. The center (or a little less) is a reasonable compromise position.  Take the same leg and move the coil out towards the end and the coil needs to get larger and larger as it approaches the end to do the same job.  IE, the coil has to be HUGE to affect nearer the end of an element.  

I've built a few 75M rotary Yagis and found there was a compromise to loading coil position and coil size/losses.  Usually  30% out on each side of a director or reflector element was a reasonable compromise.  
T
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“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

Favorite Song - Trololo thru the years:  
https://www.youtube.com/watch?v=IVqUecYGnoM
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« Reply #7 on: November 06, 2018, 07:40:48 AM »

All these references to the MFJ259 are soooo 1990s.   Check out the FAA450 from China,  more better and less money.
https://www.elekitsorparts.com/product/faa-450-antenna-analyzer-eu1ky/
It's got a color touch screen, does sweeps, and even Smith Charts.  Everything except the 18650 rechargeable lithium battery is included and it's just a slick unit, well packaged in a solid enclosure, comes with the calibration kit, etc.   You can even calibrate it to other standards than 50 ohms if you have the need, for instance in designing broadband transformers.  Mine took about a month to get here on the slow sampan but they are a reliable outfit.  Actually I bought two, one for a friend and then a few others bought them.
It won't replace my HP 8714C or the VNA2180, both of which I use for work, but the FAA-450 is definitely the tool to have around the ham shack.  Seeing what's happening with your aerial on a wide sweep makes life easier. 
Ditch the bell-bottoms and move up to the twenty first century.
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WA2SQQ
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« Reply #8 on: November 06, 2018, 08:41:31 AM »

Given my property size restrictions, and wire antenna is going to be some sort of inverted V. All feeds come from the tower. No trees means not much of any dipole.

I use the AA-230zoom - I don't know how I ever lived without it. It was a bit pricey, but I sold off some stuff that just sitting on the shelf. I told my wife, I'm just reinvesting the $$ !
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WD5JKO


« Reply #9 on: November 06, 2018, 01:59:37 PM »



There are some store bought solutions. A couple of AM'ers around Texas are using this one from Ebay:

ULTIMAX8040 DUAL BAND DOUBLE BAZOOKA
SUPER BROADBAND, EXCELLENT SWR, ONLY 103' FEET LONG

http://tinyurl.com/ybl9trga

Jim
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WA2SQQ
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« Reply #10 on: November 06, 2018, 02:07:22 PM »

I learned long ago that shortened antennas = reduced bandwidth.
The goal this weekend is to just put up two separate inverted V's
#10 solid copper should be better than the 20ga that I've been using.
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K1JJ
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« Reply #11 on: November 06, 2018, 03:23:46 PM »

Bob,

You will be happy with the two separate inv Vees.  Full-size.  There is no better antenna than a simple, conventional, coax-fed dipole. All major horizontal antenna arrays usually start with a dipole. Yeah, openwire is FB for multi-band and bandwidth, but we pay the price rowing around the bands with a tuner. (and some losses within the tuner components as well)

When the Ford/Chevy,  openwire/coax debates start, I always point out the same fact:  Look at the biggest signals on the band - AMers, contesters... the biggest signals of the bunch that slam your S-meter.  They are almost all running coax fed antennas. The point is there really is no practical difference between the two as long as the antenna is reasonably matched to the feedline - and the coax is of good size and quality, like hardline for example.

The double bazooka: I've never tried one. Though in a space-restricted situation, it could help.  But if you have the room, it has been said that the extra bandwidth (flatter swr curve)  is achieved because of the proportional extra loss from the coax being used as the radiator. It is fed like a folded dipole instead of tied together like a single wire which sets up circulating currents IIRC.  103' of (thin) extra coax = 1-3 dB or so loss depending on freq.  The extra bandwidth gained due to the loss could be calculated.  It's like when rain contaminated coax improves the overall antenna swr bandwidth, etc.
 
It all depends on your situation. The military uses those big resistor dummyload antennas for flat swr across the whole spectrum. Might be 8dB loss or more, but they eat it for ease of operation.

No free lunches.


T

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“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

Favorite Song - Trololo thru the years:  
https://www.youtube.com/watch?v=IVqUecYGnoM
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« Reply #12 on: November 07, 2018, 11:23:45 AM »

Might I suggest a CCD antenna?

Single feed point, two bands, and a better pattern than a dipole.

A lot of work, lots of dipped mica caps, but the people who've put the work in seem to like them.

--Shane
KD6VXI

Edit.  Guess not.  Space considerations.  The ccs antenna comes out more like a DEZ.
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W1ITT
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« Reply #13 on: November 07, 2018, 12:58:20 PM »

The trouble with the CCD is that it's about twice as long as a conventional dipole for a given band.  -SQQ mentioned that he had limited space in the yard. 
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WU2D
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CW is just a narrower version of AM


« Reply #14 on: November 07, 2018, 02:44:32 PM »

Run an extension cord. Yes make fun  - I still use the old RX-250.


* 255RX.jpg (429.76 KB, 4026x2573 - viewed 26 times.)
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« Reply #15 on: November 07, 2018, 07:44:02 PM »

The question is, should I feed two separate dipoles from one feed point, or make one using traps? They will be inverted V’s.

If you decide to use traps, keep in mind that the 40M trap is an added inductor on 80M: it will dramatically shorten the 80M portion of the antenna. See the Antenna book.

73,

Bill, W4EWH
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WU2D
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CW is just a narrower version of AM


« Reply #16 on: November 07, 2018, 09:28:31 PM »

I am not so sure about the trap shortening the antenna much. Depends on the trap design. Lets say you design a conventional 40M trap using a 100 pF Cap. That cap off resonance will have a reactance of 450 or so ohms at 80 Meters. The inductance would be around 5 uH.
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Jim, W5JO
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« Reply #17 on: November 07, 2018, 10:26:33 PM »

Also traps are lossy
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Steve - K4HX
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« Reply #18 on: November 10, 2018, 11:21:14 AM »

Other ways to shorten.

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

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