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Discreet antenna masts




 
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ka1tdq
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« on: October 13, 2017, 06:11:58 PM »

This is an idea for hams that don't want to advertise their antennas.  

My 75 meter dipole is an inverted V and the ends are very low to the ground.  To fix this I've built a concrete bucket at each end with a pipe sticking out of it.  It's hard to see in the photo, but you can see one in the shadow with one leg of the dipole above it.  

The mast will be 25' of galvanized fence piping.  It's lightweight, relatively cheap and slides perfectly into the 1 1/4" diameter standpipe.  

At night when I want to operate 75 meters, I can just slide the masts in and remove them before morning.  

I put a wire lock with a PVC pipe cap on each one.  I needed this to keep the kids from putting rocks in the pipe.  It also doubles as a rain guard.  That's no big deal here though because it never freezes.  

I'm still waiting for the concrete to dry, but I can't wait to try it out!

Jon


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ka1tdq
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« Reply #1 on: October 14, 2017, 10:40:25 PM »

It does work out pretty slick for getting the ends of the dipole off the ground, but the SWR goes way up.  It went from 1.5:1 in the original configuration to 4:1.  I also can't get good matches on 20 or 40 meters using the antenna tuner where I could before.  I'm guessing it's an interaction with the metal masts. 

Tomorrow, I'll go to the big box store and get a piece of PVC to fit over the top section.  Maybe 5 feet per mast will be enough of a distance to make the antenna happy again.

Jon


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ka1tdq
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« Reply #2 on: October 15, 2017, 12:32:28 AM »

I went to Home Depot and got a 48" wooden dowel for each mast.  Put the antenna back up and the SWR was much worse at 10:1.  Then I remembered that the broadcast stations wreak havoc on my MFJ antenna analyzer at night.  I put a broadcast band filter inline and things got much better.  The resonant frequency moved up to 4.0 MHz, so I had to add 2 feet to each leg. 

Now the SWR is 1.1:1 at 3885 and 1.4:1 at 3870.

I'm really glad that happened though because I added 48 inches to each pole.  Now each end is at 24 feet.

Jon


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WO4K
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« Reply #3 on: October 15, 2017, 06:30:59 AM »

Hmmm. Are the TV transmitter sites nearby? I always thought that the TVI filters were to prevent TVI from the amateur's HF transmitter, and are placed into the coax line near the transmitter. If they can also be used filter out SWR caused by nearby TV transmitting sites, wouldn't the filter need to be inserted near the antenna feed point? Otherwise there would be SWR coming back into the shack to the point of the filter. Also, let us know your SWR after raising the ends to 25'. Are the 25' pipes capable of falling onto those power lines?
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ka1tdq
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« Reply #4 on: October 15, 2017, 06:58:00 AM »

I wish they were that tall, but the only thing in danger if they fell is my kid's play set.  The masts are surprisingly sturdy, but I won't raise them if it's really windy.  99 times out of 100 though it's sunny and calm here.  

I'm getting interference from the AM broadcast band.  There's many transmitter sites around me, and the filter I use has a cutoff around 2 MHz.  It really clears things up at night.  My MFJ antenna analyzer gets wiped out, as do my SDR receivers when it's not inline (at night).  And, the filter is inline with the receiver (not the transmitter).  

The SWR is near perfect in the AM window with the antenna raised.  

That is a good point though.  I checked the antenna again (lowered) and after I added the 2' of length to each leg.  The resonant point moved down to 3.700 MHz, and the SWR is around 3.0:1 in the AM window.  So, to operate 75 meters I'll need to raise the endpoints.  

Or use a tuner.  I don't think my MFJ tuner can handle 8 FETs though, and I certainly wouldn't want to try.

Jon



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WBear2GCR
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« Reply #5 on: October 20, 2017, 09:04:29 AM »

Can't really see the whole situation from your image.

My impulse reactions:

- move
- the power lines? not good.
- consider a magnetic loop antenna, Ted K1QAR has been getting excellent results.
- do you have an HOA there? move.

Where do you live? Looks like the southwest...
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ka1tdq
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« Reply #6 on: October 20, 2017, 11:53:14 AM »

Actually, all things considered, Iím very happy with my location. I live in Phoenix and the whole region is blanketed in HOAís. You can find neighborhoods without them, but the houses are typically smaller and older.

The power lines? Yeah, they cause lots of noise. The only way for me to overcome it is with SDR software. I canít use a traditional receiver on the low bands.

My dipole is an inverted V.  The apex is at the peak of the house, which is two story. Iíve painted the coax to match the color of the house, and the ends slope down to the brick wall. Itís all very hard to see and youíd never know that Iím a ham by looking at my house.

I did want to raise the ends of the dipole without being conspicuous.  I came up with this solution for the nighttime. It actually works out very well.

Jon
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WBear2GCR
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« Reply #7 on: October 21, 2017, 09:03:41 AM »


Flagpoles are typically not prohibited, height not mentioned.  Wink
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ka1tdq
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« Reply #8 on: October 21, 2017, 09:10:01 AM »

Ha! Youíre right. I didnít think of that.  A US flag on one and an Arizona flag on the other.  Iím going to have the coolest house in the neighborhood.

Jon
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K1JJ
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« Reply #9 on: October 21, 2017, 12:51:49 PM »

Ha! Youíre right. I didnít think of that.  A US flag on one and an Arizona flag on the other.  Iím going to have the coolest house in the neighborhood.

Jon

Hi Jon,

Read this - federal US flag display laws:

https://www.senate.gov/reference/resources/pdf/RL30243.pdf


Yes, the masts are well worth it to get the average height of the dipole above ground. As you saw, raising the ends has an enormous effect on SWR, which in turn means the ground losses are greatly reduced.

But before you paint yourself into a corner by coming out with flag poles and the American flag to the neighborhood, read the US laws concerning US flags - nighttime lighting, holiday display, half-mast, raising and lowering formalities, etc., etc.  It's possible that some self-righteous prick in the neighborhood will realize your true ham intent and make it a problem unless you jump thru all the hoops.

If it were me, I would paint the masts flat black wrinkle to diffract the light and keep them up only at night. If the dipole legs are loosely attached, all you have to do is slip the masts into their holders and the ant is pulled taunt. Easy.  If the lights go on or car headlights pass by, the flat black rough-texture paint will hide the masts well at night.

Sometimes staying under the radar is the best way when in a crowded housing area.  Not to mention if they start blaming every RFI and corrosive connection problem on you - the neighborhood ham geek experimenter... especially with 100 KV++ power lines nearby, prone to their own corona, general leakage and insulator problems.

Heck, I learned living in an eight-unit apt in Denver. After one month I got an eviction notice for loud drilling noises at 3AM and interference to my neighbors. They traced the coax to my window.   

Later on in CT, my neighbor, 1/2 mile away, tried to infer I was causing his garage door to mysteriously open and close on its own randomly.  He saw the towers up on the hill and that was it.  Grin

T
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« Reply #10 on: October 21, 2017, 02:29:41 PM »



T,

"  Later on in CT, my neighbor, 1/2 mile away, tried to infer I was causing his garage door to mysteriously open and close on its own randomly. "

Well, I'm waiting for your denial .... ..

klc
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ka1tdq
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« Reply #11 on: October 21, 2017, 02:40:41 PM »

The black wrinkle is a good idea.  I'll paint the masts as well to limit night sightings.  I don't think my neighbors know that I'm a ham (despite all my weird backyard behavior), or even know what a ham is.  A commercially purchased aluminum antenna would be a dead giveaway, but they don't know what the wire's for. 

Yeah, drilling at 3am in an apartment complex is a definite no-no.  I remember being away on a week long business trip and drilling in a hotel room, hoping that nobody would complain about the noise.  I wouldn't do that now.  Start a construction project in a hotel room, next thing you know the SWAT team is breaking down your door. 

Jon   
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