Can a reflector be used with an 11 meter dipole antenna to boost performance?

Thread Starter

Darkstar

Joined Sep 3, 2010
177
Hello, I live in a condo and can't put an antenna outside for my CB radio so I have an inverted V dipole indoors. It works and I can get 1.0:1 SWR but it's indoors on the second floor of the bldg so it could do better. My radio seems to have a weak receiver so this may be a way to help it until I can fix that. I was wondering if anyone has heard of a way to use this kind of antenna with a reflector to boost performance? I've used vertical antennas before and got good results by enlarging the ground plane beneath them, but I haven't seen anything similar with any kind of dipole. I don't know if I should treat it more like a horizontal dipole or vertical since it has features of both. Either way, space will be limited. The best idea I've come up with (but can't easily fit) is to put some large sheets of aluminized mylar along one side of the antenna spaced at least 1/2 wavelength from the antenna as a vertical ground plane to boost the gain on the side opposite the mylar. My living room and kitchen would be inside the antenna!
Thanks
PS. I ruled out using a "corner reflector" because it's size would me much bigger than the dipole itself. That's why they're only used for higher frequency antennas.
 
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LowQCab

Joined Nov 6, 2012
502
An "Inverted V" is a poor Antenna Design no matter where you put it, it was designed to make possible
a really large Antenna for lower, longer frequencies, a viable possibility.
What you need is some very small gauge wire that is the same color as the outside of your building.
Make 2 pieces, each 108 inches long, at least one of them can have a small weight tied to the end,
and then simply throw it out of your window, and let it hang straight down, this will be your "Ground Plane".
Next, run another 108" wire from the bottom of the Window, to the top of the Window,
this should take up about 4 feet of that 108".
Next, you need to figure out what to do with the remaining 5 feet of wire .......
You don't want to double it back on its self, that would be the worst solution,
running it sideways along a Curtain-Rod over the Window is a possibility,
Screwing a small hook into the Ceiling to tie the end of the wire to might work just fine.

Your Impedance is going to be much higher than the "Inverted V",
so the SWR is going to be worse,
but SWR is only one "Indicator",
and does not take into account the radiating efficiency of the Antenna,
but only the "Impedance Match" to the Coaxial Cable.
So, even if you wind-up with 4:1 or 5:1 SWR,
you will "Hook-Up" to the "Air-Waves" with far greater efficiency.

Now you will probably see that there was nothing "weak" about your Receiver after all,
you've been trying to work the Air-Waves with a "Dummy-Load" for an Antenna.

With the Antenna that I just described, you will have limited range,
both receiving and transmitting,
in the direction being blocked by the structure you live in.
But you do have the bonus of not being too close to the Ground (Earth).

Here's what you actually NEED .........
.
.Ground Planes .png1-4 Wave  vs  5-8 Wave  .pngSuper Simple Reflector 1 .pngSuper Simple Reflector 2 .png
 

LowQCab

Joined Nov 6, 2012
502
I forgot to mention .............
Since you are probably "in the same room" with your Antenna,
you can pretty much do anything you want with the Coax between your Radio and "the Window".
Normally, your Coax run should be cut to lengths that are a multiple of one half wave length.
That would normally be around 18 feet, (or ~5.5 Meters),
so, you can experiment with different lengths of Coax, (less than 1/2 Wavelength).
You may find that a 1/4 Wavelength of Coax will give you a lower SWR.
I would try 4.5 feet, 9 feet, 13.5 feet, and 18 foot long pieces of Coax,
and then check the SWR reading on each one.

Lower SWR is not "Everything" or "All Important",
and while less SWR is ALWAYS a welcome bonus,
the radiating efficiency of your Antenna may be substantially more useful/valuable .
.
.
 
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Janis59

Joined Aug 21, 2017
1,332
I would say SWR indeed is not the only criterion BUT it must be reasonably low, and its much important as well to have a reactive part of impedance as low as possible. About cabling length - yepp, that is standard technique when analyzing unknown antenna with known frequency. Just other-how multiple reflections of the cable, plugs, bending, soldering will so much bulk the VNA picture that even devil with all his evil spirits will not sort out what are the spokes and which are coming from antenna. Then solution is or to use a ultrashort line shorter than lambda divide to 40 or use the coax length having no reflections, however then You cannot trust on the frequency scan VNA graph. How I try to do is switch off the antenna all matching circuits, solder VNA straight on the ends and optimize radiator lengths until the purely active resistance, means it is at resonance. Observe do the reactance goes from - to + or vice versa, means parallel or serial resonance. Resonances normally are about 10-100 pieces, thus use the widest scale to see all. Then when the right range is cached, narrow the range and set the sliding triangle to the target frequency, then cut radiator length milimeter by milimeter whilst triangle shows in the center of resonant curve. Then solder on the matching circuit and repeat all until the 50 Ohms are at the hands. Then solder the coax and repeat the check from the plug.
Available machinery: nano-VNA version v1=50 kHz to 1,5 GHz, version v2 is 50 kHz to 3 GHz. (prices +/- about 30-40 USD and 50-60 USD)
More handy in use (much more handy) gadget N1201 older version 137 MHz-2.7 GHz and modern version 37 MHz-2.7 GHz. (prices +/- about 120 USD and 160 USD).
Pictures below have only illustrative meaning

https://i.ytimg.com/vi/THc5ZTxQUSg/maxresdefault.jpg
https://hackaday.com/wp-content/uploads/2018/03/vna-e1522110535970.png
 

peterdeco

Joined Oct 8, 2019
356
I know this is against every bit of radio transmitting and receiving principles but it worked for me. I was in the same situation years ago in my apartment. I ran about 25 feet of enameled magnet wire (invisible to neighbors) out my window and up a tree on the side of the building. Totally out of resonance and wire much too thin for the output power, but I had more transmitting and receiving range than my friend with a "real" antenna on the roof of his house. Perhaps my final amp survived this because it was a tube rig.
 

nsaspook

Joined Aug 27, 2009
8,377
I know this is against every bit of radio transmitting and receiving principles but it worked for me. I was in the same situation years ago in my apartment. I ran about 25 feet of enameled magnet wire (invisible to neighbors) out my window and up a tree on the side of the building. Totally out of resonance and wire much too thin for the output power, but I had more transmitting and receiving range than my friend with a "real" antenna on the roof of his house. Perhaps my final amp survived this because it was a tube rig.
It's actually not against every bit of the radio transmitting and receiving principles of antennas. SWR and impedance matching are transmission line and circuit energy transmission requirements for low loss movement of RF energy from A to B without a reflection of RF energy from B back to A. Good impedance matches at every point in the transmission path will lower the requirements of circuits and transmission parts to withstand reflected power voltage and current peaks that cause circulation currents with possible large I/R losses. At B there still will be coupling (a gradual shift of Z of the feed-line to the characteristic Z of free space) of RF energy into space if the antenna and ground elements provide the sufficient EM phase-shift for efficient radiating without a close match if the path from source energy to antenna is low loss.

We would typically couple several kilowatt transmitters from 2 to 30 MHz using a multicoupler match into one large, high RF voltage and low loss (N2 pressurized) Heliax coaxial line hundreds of feet long from the RF shack connected to a fan of several wire-rope antennas at the end of the coax. A separate (physically separated) receive antenna of much the same fan design was also used.
fan.jpg
http://www.signalcontrol.com/products/andrew/Andrew_Coaxial_Cable_Selection_Guide.pdf

I'm not saying you shouldn't use a proper antenna specifically designed for a band like 11 meters. I just don't think adding elements will make a difference if your initial antenna installation performance is poor.
 
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Thread Starter

Darkstar

Joined Sep 3, 2010
177
To LowQCab, Janis59, bertus, Peterdeco, Nsaspook,

Thanks for all the info, opinions, and diagrams!
I ended up with this antenna because it worked better than a 1/4 wave ant mounted on a large metal picnic table. It's also better than a 1/2 wave vertical antenna with ground planes tilted to fit between floor & ceiling,

I thought I heard that dipoles were a good choice. A main prob is they are easily mounted horizontally compared to most CB antennas being vertical. The inverted V has a radiation pattern with more coming from the ends than a horizontal dipole so it is better than a straight dipole.

In some ways it would be easy to hide it outside but with the regular maintenance of grass trees, windows, painting, gutters, etc, it would soon be found.

We have rules restricting Hams, maybe because of some antenna sizes or the wattage used but the 27 MHz band is used for RC toys and there are no restrictions there about use. Mounting on the bldg is different. I am allowed to put things in the attic though.

The only place I can hide it and get more height is to put it in the attic. A vertical ant would be better than a tilted or horizontal. I could put a vertical dipole with part in my unit and the rest in the attic.

I agree with Peterdeco about the durability of these old tube radios. feel like a transistor radio would have burnt out from all the experimenting I've done. Especially this year trying to hunt down noise as well as tune antennas.

My current problem with the radio just showed up suddenly 6 days ago. The receiver will be nice and stable all day after warming up but if I transmit just for a moment, this somehow causes the receiver to suddenly have the frequencies shift all up and down the dial. Yesterday I found some voltages changing on tube pins for unknown reasons but I don't know what's causing that.

Right now that's the bigger problem so I'd appreciate it if anyone with repair experience could take a look & give me some direction. It's at :
https://forum.allaboutcircuits.com/...help-me-solve-this-sudden-new-problem.175183/

Thanks for your help!
 

Thread Starter

Darkstar

Joined Sep 3, 2010
177
YOU CAN ACCESS YOUR ATTIC ?!?!?!!!
Don't mess around, build this Antenna now ........
.
.View attachment 225771
Great, thanks, I will build it. I already have a mobile antenna to use. I had tried it held in a vice sitting on a 5' long metal picnic table. Even in front of large balcony doors (metal framed), it didn't work too well.

In high school I had a half wave ground plane on a tripod on our roof. It worked very well. Once I lined the inside of the attic roof with aluminum foil. People thought I had a linear amp. I was using the same radio I'm working on now, but it was younger then. I had to remove most of the foil because it blocked TV signals from the TV antenna inside the attic.
Thanks again.
 

LowQCab

Joined Nov 6, 2012
502
The Ground Plane is fully HALF of the Antenna.
The Vertical Radiator and the "Ground Plane Antennas" should be identical parts.
But the Ground Plane can be much larger than the Vertical Radiator.
Gains can be had with up to 3 wavelengths long "Ground Radials".
.
.
 

SamR

Joined Mar 19, 2019
3,399
You're dealing with ~4-7Watts @ 11Meters with a typical output impedance of 50Ω. You don't need large wire and even bare 20AWG at more than 50' away is pretty near invisible unless you are looking for it. Spend some time with a free online PDF of one of the ARRL antenna books for some antenna designs. Most CBs use a vertically orientated antenna for local communications so that is what you want to shoot for. Minimum 1/4 wavelength which is ~10' so to hang a vertical dipole that is ~20. 10' for the active side and 10' for the counterpoise ground leg. Longer is better. You will need a couple of insulators at the ends, a 50Ω Balun for the center-feed, and some RG-8X coax feed line from the radio to the Balun. Longer is better 5/8 wavelength or even the full-Magilla 11M full wave. Or, you could just throw a mobile base loaded whip antenna out the window when you want to use it. I'd go for the vertical dipole.
 

Thread Starter

Darkstar

Joined Sep 3, 2010
177
You guys have given me some good ideas here, thanks. It would be most difficult to put anything outside because of rules and bldg maintenance finding anything I tried to hide. Assuming I get association approval, my only options outside are
1. to run one half of a vertical dipole up one of my balcony supports and use a tripod to support the upper half, as long as it does not go higher than the top of the roof.
2. Put a small antenna like that suggested by LowQCab on the balcony either full time or temporarily, but height suffers and it is hidden by the bldg.
Nobody can get into the attic without me knowing so I can put one in there and get more height than I can outside.
3. I can again use the one suggested by LowQCab.
4. I know my half wave antenna is about 2 feet too long to be vertical, but a slight tilt is do-able.
Or,
5. I have more than enough room for a quarter wave ground plane with 90* radials. This gives me a longer antenna, more height (I could hang it from the peak of the roof and work down from there if I wanted every inch), its omnidirectional, it can be made with materials I have on hand, it can be self supporting or I can tie it to the attic beams. This is larger and more cost effective.
Finally, thinking outside the box more,
6. I can put a half wave in the attic without going into the attic by inserting a full half wave vertical through a hole in a closet ceiling. The ground planes would have to angle down to the floor then go horizontal and they wouldn't be evenly spaced due to walls and furniture and they'd be different lengths. I'd be able to remove the antenna from the attic in seconds if repairmen went in there.
7. If I could find a way to get the cable from my garage to my unit I have space above the rafters in my garage too. I already have my garage door opener wire extending vertically about 20 feet above the gnd. I can open the door from 3/4 of a block away.

I'm not too worried about causing interference. My TV, computer and old analog radio have not shown any ill effects from being so close to the current indoor antenna. I guess all the digital technology filters out unwanted signals better than 50 yrs ago when it was all analog and my radio would interfere with the neighbor's TV. In the attic my antenna would be closer to the bldg TV antenna. I haven't checked the effects on TV antenna reception yet.
 
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LowQCab

Joined Nov 6, 2012
502
A half wavelength is ~18 feet, do you really have that much space in your attic ?
Also, a 1/4 WL, or a 3/4 WL, Ground Plane Antenna will have the least tuning challenges, and lowest SWRs,
3/4 WL is the best tuning compromise, but that's ~27 feet long !!!

If you have close to ~9-feet of head-room in your Attic ( 1/4 WL ), you should simply go for a
Quarter WL Ground Plane Antenna made from cheap hook-up wire, stapled to the wood beams.
This can be done in a very stealthy manner, and left installed permanently in the Attic, and no one will care.
The worst case would be a Maintenance Man accidentally knocking it down,
or stomping on the Ground Plane Radials,
( of which, there should be a minimum of three ~9-foot long Radials, and more is always better ).

In case of inadvertent damage, just rebuild it, it should take less than ~10 minutes.
.
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SamR

Joined Mar 19, 2019
3,399
Antenna designs come and go but the dipole or end-fed is always there. I still have some dipole traps that were popular long ago for building multiband dipoles.
 

SamR

Joined Mar 19, 2019
3,399
Not a bad idea using a compressed/wound 5/8 wave mobile whip antenna. And to add to the compression, a mobile ground plane also instead of another compressed whip as the ground counterpoise. Or even a dual or array deployment of antennas.

1608854689089.png
 
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