Antenna design

[dbseeker]

As you may know from previous posts, I am not an engineer but have had basic physics courses and got interested in antennas and reception with the digital switch.
It would appear to this novice that there are two basic types of high gain antennas. Yagis representing a single horizontal plane antenna that use directors to bend and concentrate signal onto a basic active element. The second class would be those that collect signal by shear surface area as represented by the 8 bays and parabolics.
First question is why are all high gain VHF antennas of the Yagi style (with the exception of the C5)? There appears to be no modern high surface area antenna for VHF.
Second question--can anyone direct me to a novice reference on antenna design that won't require an engineering degree? I have read most of the forums and Ken Nist HDTV info with some semblance of understanding but would like to know more.
Looking forward to your replies!

[rabbit73]

First question is why are all high gain VHF antennas of the Yagi style (with the exception of the C5)? There appears to be no modern high surface area antenna for VHF.

It is possible to build a 4-bay bowtie antenna for VHF-High, but it would be three times as large as the UHF version. Antenna designs can be scaled to other frequencies. The size is an inverse of frequency, so a bowtie antenna for 200 MHz would be three times the size of one for 600 MHz.

The Gray-Hoverman antenna is about the same size as a bowtie antenna. Flint Ridge built one for VHF-High. See attachment.

A manufacturer isn't going to make one unless they can sell it.

Second question--can anyone direct me to a novice reference on antenna design that won't require an engineering degree?

My favorite reference is the ARRL Antenna Book (not ARRL Handbook). You can borrow one from your local library, any recent edition. If they don't have it in their collection, their reference desk can borrow one interlibrary. If you like it, have someone give it to you as a present.

[dbseeker]

Additional questions-- What determines optimum director diameter and how was that derived?
When ganging antennas for additional gain, the second adds an additional 2.5 to 3 db of gain. So does it require 4 ganged antennas to increase another 2.5 db of gain and on up exponentially to keep increasing at a rate of 2.5 db of gain?

[rickbb]

Check out this page by forum member holl_ands.

http://imageevent.com/holl_ands/omni...rqgik1.goose_s

He has some VHF designs using high surface area. Trapezoids called the goal keeper, and even a single whip antenna for VHF.

[ADTech]

Additional questions-- What determines optimum director diameter and how was that derived?

bandwidth and cost/mechanical load concerns

When ganging antennas for additional gain, the second adds an additional 2.5 to 3 db of gain.

A "perfect", no loss combination would yield up to 3 dB of gain, a doubling. Want another (theoretical) 3 dB of gain? Double the antenna again. And so on. In the real world, one soon runs into real size and weight constraints as well as cumulative combining losses that will soon result in a point of diminishing returns.

[dbseeker]

thank you for all the replies!!
Size is the inverse of frequency hence the size of the VHF-hi GH evidenced by the photo.
Optimum director diameter is determined by cost/mechanical load and bandwidth.
The DIY antennas don't seem to focus on wire size and in areas of strong signal, a paper clip can interestingly suffice. May I conclude that antenna design is far more influential in achieving gain and diameter size of directors and /or active element minimal or negligible ?
Please comment on a hypothetical antenna setup that may tie in my questions ( or maybe not)
Assume a 50 foot mast or tower that is 25 foot above roof line so there is 25 feet of available signal capture.
A multi-folded dipole is configured with a horizontal length corresponding to the current hi-VHF director length that then drops vertically at 90 degrees for a 3 inch distance similar to "closing the gap" on 8 bay mods for UHF. A one inch gap is at each fold of the dipole. The array ends in a standard balun with a contiguous multifolded dipole with no phasing.
Can a dipole be folded infinitely (within reason) , optimize surface area for signal capture and gain, and be effective for both UHF and VHF?
What factors have I missed or minimized?
Have excluded weight, aesthetics, cost, wind resistance--just a hypothetical.

[rabbit73]

The yagi antenna has a bandwidth that is much less than a bowtie antenna, because the yagi is a high "Q" antenna as compared to the bowtie antenna which has a low "Q," as defined by the max allowable SWR over the required bandwidth. The bowtie antenna is a member of the collinear array family, which you will read about in the ARRL Antenna Book.

Here you see the collinear array with straight wire elements. The >< shape "whiskers" make the bowtie antenna even more broadbanded.



When designing a Yagi the element length and diameter must be carefully controlled for max gain at the desired design frequency. Each element must be resonant at a certain frequency. The director elements are shorter than the driven element, and the reflector element is longer. Larger diameter elements must be shorter for the same resonant frequency, and larger diameter elements make the yagi more broadbanded to cover a wider frequency range.

Computer modeling will give you the optimum element length, diameter, and element spacing for a yagi; much better than trial-and-error.

Please comment on a hypothetical antenna setup that may tie in my questions ( or maybe not)

Is there a hypothetical tvfool report for your hypothetical antenna?

A multi-folded dipole is configured with a horizontal length corresponding to the current hi-VHF director length that then drops vertically at 90 degrees for a 3 inch distance similar to "closing the gap" on 8 bay mods for UHF. A one inch gap is at each fold of the dipole. The array ends in a standard balun with a contiguous multifolded dipole with no phasing.

I'm having a very hard time picturing your hypothetical antenna in my mind. Can you show us a sketch or diagram? Is this a yagi or bowtie antenna? What is a multi-folded dipole? Something like this?

Can a dipole be folded infinitely (within reason) , optimize surface area for signal capture and gain, and be effective for both UHF and VHF?

No, when folded you reduce capture area, reduce the sections that are horizontal to match polarization, reduce gain and are less likely to work on VHF and UHF, if I understand your question.

Are you thinking of a fractal antenna?

https://en.wikipedia.org/wiki/Fractal_antenna

https://www.google.com/?gws_rd=ssl#q=fractal+antenna
https://www.google.com/search?q=frac...xfArf&dpr=1.25

http://www.digitalhome.ca/forum/186-...velopment.html

or are you thinking of one antenna for UHF and VHF?
http://www.digitalhome.ca/forum/186-...provement.html

http://clients.teksavvy.com/~nickm/

[dbseeker]

thanks for the replies!!
Was able to order to order the handbook through Amazon used for $32 including shipping.
Please pardon the crude, kindergarten sketch done hastily but may give you a flavor for what I was trying to describe.
One contiguous loop with a 33inch top dropping at 90 degrees for 3 inches and returning horizontal for 16 inches with one inch gap in between. 16 inches is the current director length I have measured for combo 7-51 antennas on the VHF side. The 3 inch distance arrived at as distance between the tips of UHF array and are closed in on 8 bay mods. The balun added at the bottom when done with the amount of repetition that you desire. Lots of surface area but have no clue about any or all design ramifications. Constructed one with five repetitions using small caliber copper wire and put in my attic. Receives all local channels but signals are strong at 25 feet AGL from 44 to 74 db.
Many thanks for indulging this novice.

[rabbit73]

Thanks for the sketch. It gives me a good idea of what you are doing.

I've never seen anything like that before, but understand why you call it multi-folded dipole. Glad to hear you are getting good results, but most any antenna will work in a strong signal area.

Someone who is good at computer modeling could analyze your antenna for performance. I don't do modeling but use a test setup to compare a new antenna with a know standard antenna using a steady OTA signal and an A/B switch to rapidly change between the two antennas and get signal strength readings on my signal level meter that is calibrated in dBmV.

Constructed one with five repetitions using small caliber copper wire and put in my attic. Receives all local channels but signals are strong at 25 feet AGL from 44 to 74 db.

May I see a tvfool report for your location?

What are you using to give you those dB readings?

[rabbit73]

I put labels on your diagram; please check my work.

[dbseeker]

Please find attached pic of the antenna in the attic. Actually made of aluminum wire and certainly not made with extreme tolerances. Each vertical distances and gaps are all 3 inches. Other than the top contiguous length, all other horizontal sections are 16 inches and the very bottom with balun attached. Have it fed to a Samsung plasma tv in garage with just vertical bar signal strength meter. Unfortunately nothing more precise.
Posting tv fool report shortly.
Lowest channel 47 at 44 db rates 7 out of 10 bars channel VHF real 7--- rates 7 out of 10 bars
Real channel 9 --10 out of 10 bars

[dbseeker]

Label A is 3 Inches
http://www.tvfool.com/?option=com_wr...8e03b729885895

[rabbit73]

Thanks for the correction, the photo of your creation, and the tvfool report.

Have it fed to a Samsung plasma tv in garage with just vertical bar signal strength meter. Unfortunately nothing more precise.

That scale is a lot better than nothing, and good enough to make comparisons.

Lowest channel 47 at 44 db rates 7 out of 10 bars

Oh, OK, I get it; the 44 dB comes from the Noise Margin reading on the report.

[dbseeker]

For grins, I thought I would try to navigate the software simulators although not sure what I would encounter. Do maybe 5 repetitions and 10 and 15 or if too difficult--just forget it all and go to bed.
will try to navigate the posts on accuracy of simulators and real world efficacy.
Thanks for all your time and thought. Enjoy the thought processes!

[dbseeker]

Please tolerate a question or two about reflectors. Optimum reflector for UHF antenna is 3 inches to add close to 3 db of gain according to what I can ascertain from this forum and HDTV primer. Is it safe to assume that the additional 6 inches the wave travels to and from the reflector to the active element is negligible in relation to the distance from the transmitter? At what reflector distance does out of phase signal occur? To me, at least intuitively, the shorter the reflector distance, the less likely the reflected signal would be out of phase.
On a related note, I have a antennacaft Y7-13 and the active elements on the antenna are 12 inches apart and linked together with the balun. I assume both collect signal and are additive in signal capture. Is there a wavelength formula that determines maximal signal strength based on active elements, distance, and reflectors?
thanks!!

[rabbit73]

Optimum reflector for UHF antenna is 3 inches to add close to 3 db of gain according to what I can ascertain from this forum and HDTV primer.

I assume you mean distance between the reflector and bowtie elements. The distance is a function of frequency. The original Channel Master 4221 had the reflector 4.25 inches behind the elements. For the present 14-51 UHF band, optimum spacing is about 4.5 to 4.75 inches. As the FCC takes away more of the UHF band and gives it to cellular interests, the optimum spacing will increase as will the element length. Think in terms of 0.2 to 0.25 wavelength.

Is it safe to assume that the additional 6 inches the wave travels to and from the reflector to the active element is negligible in relation to the distance from the transmitter?

Yes.

At what reflector distance does out of phase signal occur? To me, at least intuitively, the shorter the reflector distance, the less likely the reflected signal would be out of phase.

There is an optimum distance that is just the right distance away.

You want the energy from the reflector to arrive at the driven element to add to the energy in the driven element in phase. Your intuition is correct, the wave front does take a little longer to arrive at the reflector and then travel back to the driven element, but it doesn't seem to matter as long as the two energy sources are in phase. In a reflector antenna, the adjustment is made by changing the distance. In a yagi the adjustment is made by changing the distance and the reflector length.

In Bill Orr's Beam Antenna Handbook, he gives an interesting explanation of how a yagi parasitic element works. The incoming wavefront induces a current in the driven element, the reflector, and the director/directors. Since energy can not be created or destroyed, the energy in the parasitic elements has to go somewhere. It is re-radiated by the parasitic elements and coupled into the driven element to add to its energy, giving the desired increase in gain.

I have a antennacaft Y7-13 and the active elements on the antenna are 12 inches apart and linked together with the balun.

IIRC, the Y10-7-13 and Y5-7-13 are a log periodic design that is different than a yagi to cover the whole VHF-High band. Look in your ARRL Antenna Book for LPDAs.

[GroundUrMast]

Is there a wavelength formula...

The basic formula to convert frequency into wave length is 299.8 Meters/Frequency (MHz)

For example, UHF channel 14 is 470 MHz to 476 MHz. Therefor, if you want to know the wave length at that channels center frequency (473 Mhz) you would divide 299.8 by 473, the result being 0.634 Meters (rounded to three decimal places).

[GroundUrMast]

On another note... I tossed a simulation of your earlier antenna (post #10 of this thread) together, using 4NEC2

CM 
CE
SY AGL=0
GW	100	33	33/2	0	0+AGL	-33/2	0	0+AGL	#16
GW	101	3	33/2	0	0+AGL	33/2	0	3+AGL	#16
GW	102	3	-33/2	0	0+AGL	-33/2	0	3+AGL	#16
GW	103	16	33/2	0	3+AGL	0.5	0	3+AGL	#16
GW	104	16	-33/2	0	3+AGL	-0.5	0	3+AGL	#16
GW	105	3	0.5	0	3+AGL	0.5	0	6+AGL	#16
GW	106	3	-0.5	0	3+AGL	-0.5	0	6+AGL	#16
GW	201	16	33/2	0	6+AGL	0.5	0	6+AGL	#16
GW	202	16	-33/2	0	6+AGL	-0.5	0	6+AGL	#16
GW	203	3	33/2	0	6+AGL	33/2	0	9+AGL	#16
GW	204	3	-33/2	0	6+AGL	-33/2	0	9+AGL	#16
GW	205	16	33/2	0	9+AGL	0.5	0	9+AGL	#16
GW	206	16	-33/2	0	9+AGL	-0.5	0	9+AGL	#16
GW	207	3	0.5	0	9+AGL	0.5	0	12+AGL	#16
GW	208	3	-0.5	0	9+AGL	-0.5	0	12+AGL	#16
GW	301	16	33/2	0	12+AGL	0.5	0	12+AGL	#16
GW	302	16	-33/2	0	12+AGL	-0.5	0	12+AGL	#16
GW	303	3	33/2	0	12+AGL	33/2	0	15+AGL	#16
GW	304	3	-33/2	0	12+AGL	-33/2	0	15+AGL	#16
GW	305	16	33/2	0	15+AGL	0.5	0	15+AGL	#16
GW	306	16	-33/2	0	15+AGL	-0.5	0	15+AGL	#16
GW	307	3	0.5	0	15+AGL	0.5	0	18+AGL	#16
GW	308	3	-0.5	0	15+AGL	-0.5	0	18+AGL	#16
GW	401	16	33/2	0	18+AGL	0.5	0	18+AGL	#16
GW	402	16	-33/2	0	18+AGL	-0.5	0	18+AGL	#16
GW	403	3	33/2	0	18+AGL	33/2	0	21+AGL	#16
GW	404	3	-33/2	0	18+AGL	-33/2	0	21+AGL	#16
GW	405	16	33/2	0	21+AGL	0.5	0	21+AGL	#16
GW	406	16	-33/2	0	21+AGL	-0.5	0	21+AGL	#16
GW	407	3	0.5	0	21+AGL	0.5	0	24+AGL	#16
GW	408	3	-0.5	0	21+AGL	-0.5	0	24+AGL	#16
GW	501	16	33/2	0	24+AGL	0.5	0	24+AGL	#16
GW	502	16	-33/2	0	24+AGL	-0.5	0	24+AGL	#16
GW	503	3	33/2	0	24+AGL	33/2	0	27+AGL	#16
GW	504	3	-33/2	0	24+AGL	-33/2	0	27+AGL	#16
GW	505	33	33/2	0	27+AGL	-33/2	0	27+AGL	#16
GS	0	0	0.0254
GE	0
GN	-1
EK
EX	0	100	17	0	1	0	0
FR	0	0	0	0	584	0
EN

The impedance is quite high making for a very high SWR.

Also, the pattern does not project front and rear as one would expect a panel antenna to do. Rather it has 4 lobes pointing downward... (see the attached 3DView which is from a perspective below the antenna.)

[rabbit73]

Originally Posted by dbseeker

To me, at least intuitively, the shorter the reflector distance, the less likely the reflected signal would be out of phase.

I was concerned about a similar problem with a 4-bay UHF bowtie antenna. As the wavefront reaches the antenna, current is induced in the elements of all four bays at the same time. However, the current from Bay No.1 has to travel along a half wave section of the phasing line before it reaches Bay No. 2. The current changes direction every half wavelength, so the phasing line is reversed to compensate for that, but isn't the current arriving too late?

The answer to the dilemma is that the currents rise, fall, and change direction in all the elements at the same time, which is another way of saying that they are in phase.



http://www.m4antenna.eastmasonvillew...0examples.html

[rabbit73]

Originally Posted by dbseeker

Is there a wavelength formula that determines maximal signal strength based on active elements, distance, and reflectors?

The formula that I use as a first approximation for a half wavelength for VHF and UHF is 5540/Freq in MHz with the answer in inches.



Example:
Channel 7, 174 to 180 MHZ, center freq 177
5540/177 = 31.3 inches