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Discussion Starter #1
Hi from Australia
I have been reading this forum for a few days trying to build a antenna that is compact enough to store in the van that covers the Australian tv VHF UHF bands.
My current antenna is a matchmaster 01mm-de7 combined 4 element UHF and folded dipole VHF element with 25db booster.
Thinking of building holl_ands UHF 2-Bay SOLID Triangles bowtie with grid reflector (Grid more robust to store in caravan) https://imageevent.com/holl_ands/multibay/uhf2baysolidtrianglesvarrrs
I'm having trouble understanding the high SWR figures and if they are detrimental to the performance of the antenna.
My understanding is to tune SWR to under 2.5 or less.
Also i noticed the feed spacing is 4in where most of the other bowtie antenna's are 1.5in.
I assume Feeder spacing affects antenna impedance and SWR
Any suggestions would be appreciated
 

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a.), b.), c.) UHF antennas at that link all have decent VSWR as indicated.


I don't want to speak for him, but quite often when holl_ands does an analysis of an antenna, he will Sweep the VHF Band as well as the UHF band. This doesn't mean the antenna was designed for VHF at all. So don't let that confuse you.
 

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Discussion Starter #3
Thanks majortom
Hollands mentioned "SWR (300-ohms) = 86 to 38 is Excessive"
So is a decent figure for that antenna
 

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As I said, look at what yoiu are reading about! eg "UHF 2-Bay SOLID Triangles - Var RR's"


So, since you are reading about a UHF antenna, ignore everything you see about VHF. Why??? Because the antenna yoiu are looking at is designed for UHF, not VHF. Simple as that...


You do know the difference, correct?
 

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Discussion Starter #5
I do know the difference between VHF and UHF antenna's and that they are designed for particular bands.
I was considering the bowtie design because of their bandwidth and ability to receive some VHFhi channels at reduced gain.
The 2 bay bowtie seems to be more compact than a Single Bay Gray-Hoverman with NAROD's or a VHF/UHF yagi dipole or phased array antenna.
I'm looking for an antenna that has reasonable bandwidth/gain for it's physical size.
Considering antenna physical size is governed by wavelength that it is designed to receive.
I have been using this antenna https://www.matchmaster.com.au/digital-tv-antennas/01mm-de7/ but would like to build one with more gain.
Just asking what affect the SWR has on the net VHF gain and what the affect feeder spacing has on antenna matching as this enclosed bowtie has 4in feeder spacing where most of the other bowties are 1.5 to 2.5in.
 

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You might want to take a look at the M4 without reflector. It's lighter and easier to build than the GH and has about 5dB gain on VHF. I used silver solder and #10 wire make mine which is light and durable. Adding a 30" wide reflector increases VHF gain considerably. The no reflector model won't be great for DX but it does a good job with fairly strong signals and doesn't require precision aiming.
 

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Hollands mentioned "SWR (300-ohms) = 86 to 38 is Excessive"
So is a decent figure for that antenna
It may help to read some wiki entries on VSWR...
https://en.wikipedia.org/wiki/Standing_wave_ratio

In a nutshell, the lower the VSWR the better. Is not critical, that vswr be great in the receive direction. Meaning, poor VSWR is not gonna shut your receiver down. However, it is a sure sign that the antenna was not designed for the band with high VSWR and probably won't work very well if at all.

In the Transmit direction VSWR is extremely critical to ensure an efficient transmitter system. Typical numbers for a Transmitter may start to limit the PA output power once it sees 2:1 VSWR, and may shut itself down entirely beyond 3:1 VSWR. Protection Circuit designs may vary depending on the type of antenna it's designed for, so on... But gives ya a rough idea..
 

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If it was me, I would go with the UHF antenna ur planning on for UHF, and for VHF I'd use a separate antenna like a pair of Rabbit Ears. Combining them with a VHF/UHF diplexor (UVSJ). That gives you more flexibility since you can orient the UHF and VHF antennas independently for optimum reception of both bands simultaneously.
 

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Start simple, see if that works for you, before heading to more complex antenna systems is what I am saying...


The missing 'Edit' button is really annoying...
 

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I'm a ham radio operator. Yes, as Majortom noted VSWR is critical if you are transmitting. As far as receiving goes, don't get hung up on VSWR.
 

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Discussion Starter #11
I worked for Telecom Australia for 30yrs, back when radio telephones transmitters systems didn't have reflected power protection or PA stage limiters. Power up without load/antenna blown TX.
I was not sure on the affect of SWR on receivers.
From holl_and design result the high RX SWR seem to have some influence the net gain of the antenna.
But as Jorget says i shouldn't be too concerned about excessive receiver SWR.
For simplicity and small size I'll give the 2 bay bowtie a go. maybe solid or 6 whisker with reflector.
Still wonder why some bowtie's have 4in feed spacing and others have 1.5 to 2.5 in spacing.
I suppose it's what the design program optimises the antenna too.
And as ExDilbert said and i have read a reflector will improve VHF gain.
I have some smaller spacing mesh 25mm x25mm for my reflector.
Thanks for your input.
 

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It's my understanding that higher SWR numbers won't hurt your receiver, like it would with older transmitters(most modern transmitters have protection circuits, that shut them down if SWR is excessive). SWR number above 1.0 do affect the actual gain of your antenna. If you look at Yurii's Gain charts, you will see Total Gain figures in blue, and Actual Gain figures charted in red. It looks to me that an SWR of 2.0 caused a loss of about 0.5db. I know that there are formulas for figuring out this number, but I think that, for most people modeling and building antennas, the figure of 2.5 or 2.7, as a max SWR, will usually ensure that your antenna will work acceptably. Always keep in mind, that SWR closer to 1.0 will have less loss of gain, at that frequency. Excessive SWR numbers are going to indicate, that you may not get anyway near the gain listed, for that frequency or band.
 

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that shut them down if SWR is excessive
that is important only for powerful rigs, which cannot handle doubled signal voltage. To keep price low, powerful transmitters always use as small transistors and tubes as possible. No one build 100 ton Caterpillar to handle 20 ton commodities.
In worst case scenario (no load at all or short circuit) SWR is getting endless and Voltage is doubled. If anode or collector can handle this double signal voltage - it will work fine for ages.

Also this is true only if transmitter has no antenna tuner (no variable impedance match), only fixed impedance 50 Ohm


I know that there are formulas for figuring out this number
https://docs.google.com/spreadsheets/d/1qOWcKIHILoxgCGVUX1DDekGm3OsjRqcEICleeDgIM1c/edit#gid=1398883485

this formula accounts only mismatch loss due to reflection. It will be true in case of zero-length transmission line (no cable, direct LNA mount) or lossless transmission line (short air line with good wire/pipe gauge)

Datasheet cable losses are always measured @ SWR=1
If standing waves occur in any cable - losses will go higher. This formula is more complicated and rarely used:
https://docs.google.com/spreadsheets/d/1GS7wS2ZRSgJXRn_z7kV58lhk3V33WNiCL-G5nbcx0MI/edit#gid=402116968

If cable has 3 dB loss (about 20 meters of TV cable at UHF), going to SWR=2 gives extra 0.4 dB heat penalty (roughly equivalent of 2 meters extra cable length)
 

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higher SWR numbers won't hurt your receiver
there are hardly any true 75 Ohm LNA which can have SWR=1 anywere at VHF or UHF frequencies.

Wideband LNA with SWR<1.5 in 174-862 MHz is hardly theoretically possible only with HEMT transistor specially designed for this frequencies to have channel impedance specifically close to 75 Ohm in 200...700 MHz range.

Most (if not all) actual TV LNA (not narrowband tunable) are built around conventional BJT or FET transistors. They have either very low input impedance (in 15...25 Ohm range), quite big reactance (j10...j15 range, which is in order of magnitude of real part of impedance) or very high (FET in common drain mode). There is no mode for conventional transistor with input impedance close to 50 or 75 Ohm.

Datasheet Nf for transistor are given for best-matching impedance (it's not always SWR=1, lowest possible Nf are often at a higher SWR, may be 1.3 ... 1.6 range)

LNA design with low noise figures is all about matching techniques.
There is no way to match 15-25 Ohm transistor to achieve SWR<5 in 174-862 MHz range

That's why best available TV tuners have only 4...5 dB noise figures (DVB-T2 standard imply not worse than 7 dB Noise figure).
Low-cost transistors with Nf=1 dB and less are available from 1990, and TV tuners or TV LNA built around this Nf=1 dB transistors get only 4 dB noise (best rigs)

This 3 dB loss is due to very poor SWR matching.

If your antenna simulator (or real-world SWR measurement) says antenna have SWR=2.7, actual SWR with actual LNA might be better or worse than 2.7
If your antenna have Z=25 +j0 and SWR75 is predicted to be 3.0 and you attach this antenna to Z=25 +j0 LNA - there won't by any reflection and SWR=1 (for direct mount).
If you have cable between LNA and antenna - there will be standing waves inside this cable (some extra 0.4..0.6 dB heat loss in 3.0 dB lossy cable) but not mismatch loss at the end (at cable-LNA junction)

if your antenna have Z=225 +j0 and SWR75 is predicted to be 3.0 and you attach this antenna to Z=25 +j0 LNA - SWR will be 9, not 3. And SWR=9 is 4.4 dB mismatch loss!
 

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Discussion Starter #16
The reason i was looking at the SWR figures because they are a lot higher in the HiVHF band on the bowtie antenna that i was thinking of building.
I see the bowtie Yurii posted the link for has very low SWR in the UHF range but i assume a lot higher in the HiVHF range.
After looking at the huge variety of antenna designs holl_day has, i found a couple of Hi VHF UHF 2 bay bowties that have a Rectangular Loop Resonator that reduces the Hi VHF SWR to an acceptable level and are compact enough to store in the caravan.
These antenna don't have a reflector fitted. So I'm trying to learn how to use the 4nec2 software to simulate one with a grid reflector fitted.
Around Australia almost all free to air tv transmitter transmit all channels from the site, so narrow high gain antenna would suit me best

Yurii thanks for your explanation on antenna receiver matching. Been too long time since i worked with polar / J notation in active circuits,
If i have to use a masthead amplifier/LNA i will be mounting it as close as possible to the antenna.
So the standing wave may have less effect when the antenna feed is shorter than operating wavelength and terminated into the amp.
 

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I'm sure that forcing UHF antenna to work as VHF antenna is not viable from any point of view: price, size, weight, perfomance.

SWR is not the only reason for perfomance degrade. When element width become less than lambda/2 it acts as director, not reflector.
If screen with is less than 860 mm @ 174 MHz - it acts as director and direct radiation in opposite to expected direction

Most lightweight and compact UHF+VHF combo is VHF dipole + UHF yagi + frequency diplexer (combiner)
 

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Discussion Starter #18
I'm using a combo VHF dipole and UHF yagi with 15db maskhead amplifier at the moment

I think this antenna has a gain of only 3dbi VHF and 5dbi UHF.
I'm learning how to use the nec program and have modeled holl_der HiVHF+UHF RL-M2 2-Bay+Loop - NO Refl and added a reflector.
The 2x4 reflector gave me mixed results.
hi VHF SWR is around 40 with reflector 4.5 in behind.
When optimised i get good hi VHF SWR when reflector is 14in behind.
Not sure what I'm doing, so may stick with the VHF/VHF combo that i already have.
 

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Not sure what I'm doing, so may stick with the VHF/VHF combo that i already have.
It's easy to add length to Yagi and get any directivity in range 5...13 dBi (proportional to length)

Any dipole is ~2 dBi. VHF dipole is not exception. To get any directivity at VHF (D>2 dBi) you need either reflector or director.

Dipole at the photo have poor perfomance for several reasons:
* it is shorted dipole, hence narrowband and R much lower than 300 Ohm. I expect SWR at least 6 over the band and realized gain -2...-1 dBi (less than 0 dBi)
* it uses open symmetrical transmission line, which is traced faulty. Both wires in symmetrical line have to be equally coupled to any obstacles (ground) and coupling to obstacles should be much less then coupling between wires. This is not true on photo. You can not wrap open symmetrical line over ground metal boom.
Due to this I expect huge directivity distortion (boom radiation) and impedance imbalance (feed radiation) and further mismatch.
 
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