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Author Topic: VHF/UHF height vs. feedline loss question  (Read 687 times)

KD0VE

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VHF/UHF height vs. feedline loss question
« on: March 08, 2022, 08:35:24 AM »

I have little experience with VHF/UHF but developed a modest station during Covid.

I will be lowering my 70' tower this spring to install a new HF beam.  Not something I will do very often.

Thinking about options for getting a VHF/UHF antenna up there.  However, the feedline, which is RG213, is about 150' so over 3dB of loss minimum.

My current VHF/UHF antenna is a coaxial vertical (AR270B) at about 30' fed with LMR400 and it works pretty well hitting repeaters over a wide area.

I'm questioning if it's worth the effort to add a VHF/UHF (2M and 70CM) on the tower.  Running an additional, lower loss feedline from the tower base to the shack is not an option.  Remote antenna switch is an Ameritron RCS-8V which looks OK for 2M, 70CM questionable.

thx to anyone taking the time to offer advice.
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G4AON

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Re: VHF/UHF height vs. feedline loss question
« Reply #1 on: March 08, 2022, 08:47:33 AM »

150’ of RG213 on VHF/UHF is too much loss. While height helps, so does minimising loss by using better coax and mast head preamps.

This free book should help a lot.

http://www.trpub.net/html/dx_book.htm

73 Dave
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WA3SKN

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Re: VHF/UHF height vs. feedline loss question
« Reply #2 on: March 08, 2022, 10:03:44 AM »

For FM/repeater operations, if you are hitting the repeaters you want OK then there is no need for more height.  If you are planning "weak signal VHF/UHF" it would probably be worth getting things higher.

-Mike.
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KM3F

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Re: VHF/UHF height vs. feedline loss question
« Reply #3 on: March 08, 2022, 11:50:39 AM »

7/8" Heliax feed line and higher gain antenna will do the trick.
Coax switch needs to be one for the Freq. range above 1000 mhz with N connection to be worthwhile. They are not that expensive if you really want better performance.
The feed line alone is worth the gains from lower loss over the length you specify.
From experience, here.
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AA4PB

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Re: VHF/UHF height vs. feedline loss question
« Reply #4 on: March 08, 2022, 12:31:35 PM »

A mast mounted preamp can make up for feedline loss on receive. More transmitter power output can make up for feedline loss on transmit.
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Bob  AA4PB
Garrisonville, VA

K1KIM

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Re: VHF/UHF height vs. feedline loss question
« Reply #5 on: March 08, 2022, 01:13:23 PM »

7/8" Heliax feed line and higher gain antenna will do the trick.
Coax switch needs to be one for the Freq. range above 1000 mhz with N connection to be worthwhile. They are not that expensive if you really want better performance.
The feed line alone is worth the gains from lower loss over the length you specify.
From experience, here.

Running an additional, lower loss feedline from the tower base to the shack is not an option.
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W9FIB

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Re: VHF/UHF height vs. feedline loss question
« Reply #6 on: March 08, 2022, 02:18:52 PM »

7/8" Heliax feed line and higher gain antenna will do the trick.
Coax switch needs to be one for the Freq. range above 1000 mhz with N connection to be worthwhile. They are not that expensive if you really want better performance.
The feed line alone is worth the gains from lower loss over the length you specify.
From experience, here.

Running an additional, lower loss feedline from the tower base to the shack is not an option.

Then the rest doesn't matter.
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73, Stan
Travelling the world one signal at a time.

N8AUC

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Re: VHF/UHF height vs. feedline loss question
« Reply #7 on: March 08, 2022, 02:49:13 PM »

This is something I've thought about from time to time.
And I think this is where antenna gain comes into play.
The reason we use decibels (dB) in the first place is to make the math easy.
Let's say you've got feedline with 3dB of loss, and you've said you can't
change that, so you're stuck with it for now.

If you put a 1/4 wavelength antenna at the end of that, you're net -3dB (antenna gain + feed line loss)
But if you put an antenna with 3dB gain at the end of that, you net to 0dB.
If you put an antenna with 6dB gain at the end of that, you net to +3dB...and so on and so forth.

Now what's the value of that? Depends on what you want to be able to do.
If you're just doing FM and repeaters mostly, and you can already hit what you want, then not much value added.
But if you want to do FM simplex, or weak signal work, then there could be value added.
Only you can determine if the juice is worth the squeeze.

Hope this helps you to ponder your options.
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W9IQ

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Re: VHF/UHF height vs. feedline loss question
« Reply #8 on: March 08, 2022, 03:52:48 PM »

I would use some coverage modeling software to see what the additional height minus the coax loss does for you. Here is a nice site that is totally free that I have used for several repeater installations: https://www.ve2dbe.com/rmonline_s.asp. You can easily model the two situations with their respective antenna gains, coax losses and antenna heights and compare the results.

- Glenn W9IQ
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- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

WB6BYU

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Re: VHF/UHF height vs. feedline loss question
« Reply #9 on: March 08, 2022, 05:54:51 PM »

Quote from: KD0ZGW

Thinking about options for getting a VHF/UHF antenna up there.  However, the feedline, which is RG213, is about 150' so over 3dB of loss minimum.



So the first question is, does the current antenna go through
the same 150' of coax and the switchbox?

If so, then the loss in the cable is a constant, and doesn't factor
into the comparison.  If not, then we need to figure the total
difference in coax losses as part of the calculations.


WB2WIK used to say that no feedline was too long if it ran
vertically.  That is, the gain due to height is more than the
losses in any reasonable feedline.

I have some sample numbers for relative gain over a 10 mile
path on 2m, but they only go up to 40'.  I looks like the gain
is about 2 dB going from 30' to 40'.  Even RG-174 would still
show a net gain at that point (it has about 1 dB loss in 10').

That's not to say that you should use lossy coax: use the lowest
loss type that fits your budget.  But the added gain due to
height will often outweigh the loss of the (vertical) coax.


But there are other factors to consider, too:  your operating
preferences, local terrain, and local repeater locations.

I live in a valley, and my primary limitation is topographic.  I have
good coverage of surrounding repeaters (mostly on the surrounding
hills) with a low antenna, but I can't raise it far enough to clear the
surrounding hills.  So a higher antenna, or higher gain, won't make
much difference on many paths.

If you are already on a 5000' hill, the net change in height may be
insignificant (once it is high enough to clear the edge of the hill).

But over flat ground, tower height can make a big difference, and
you probably would want to do some modeling to see how much it
improves your signal into other area repeaters (or to local towns
with simplex activity).

That, of course, assumes that you are operating FM, which is
likely given your current antenna.  For weak signals modes
like CW or SSB, height is even more important, and you'd
want a horizontally polarized antenna, probably a beam for each
band.


Quote

My current VHF/UHF antenna is a coaxial vertical (AR270B) at about 30' fed with LMR400 and it works pretty well hitting repeaters over a wide area.



The AR270B is a "collinear antenna", but not a "coaxial antenna".
It's the same principle as the old Ringo models, but the old ring
has been replaced with a sealed pre-tuned matching section when
they converted it to dual-band operation.

Probably would have better decoupling on 2m if the radials were
19" rather than 6 1/2", but that seems to be a change that a lot of
manufacturers are making on their dual-band antennas to save cost.

K5LXP

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Re: VHF/UHF height vs. feedline loss question
« Reply #10 on: March 14, 2022, 05:54:42 AM »

Mentioned but I don't think emphasized enough is the additional coverage one would get from the increased height.  Even if you had 6dB of end to end loss, you would still get more range (though less signal density).  I would skip the switch/relay box, I use diplexers on both ends.  While a set of these is not inexpensive (4ea) it allowed me to afford one run of low loss coax.  The net result is one run of hardline and operating three bands at once through it, and no switching.  There's no reason to not keep the existing 30' antenna, I have both tower and roof mount antennas and use them interchangeably.  I would certainly use the opportunity of the tower being serviced to do *something*, even if it's not "ideal".  Better to hang it and have a chance to do something than not, and have no chance.

Mark K5LXP
Albuquerque, NM
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K0UA

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Re: VHF/UHF height vs. feedline loss question
« Reply #11 on: March 14, 2022, 12:36:36 PM »

Forget the higher 2M 70CM antenna on your tower, and put up a simple horizontal 6 meter antenna like a Par omniangle or like I have the simple and lightweight Par Moxon for 6 meters (see my QRZ page) This will give you a capability you do not now have, and will work with your coax switch, and is great fun during the summer E skip season. The 2M 70CM is working fine for your FM needs now, but you have no 6 meter capability now and with small money, small wind load, and small effort, you have additional capability.   Just a thought.
Oh, if your new HF beam already contains 6M capability then forget everything I just said. :)
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73  James K0UA

KM3F

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Re: VHF/UHF height vs. feedline loss question
« Reply #12 on: May 01, 2022, 01:06:55 PM »

height vs feed line loss:
Examples;
Antenna has gain of 5 dB, Feed line loss is 3dB. Net gain is 2db plus the height for signal gain.
Antenna has gain of 6 dB, Feed line loss is 6dB. Net gain is 0 plus the height for signal gain.
The last one is the practical limit unless a low noise pre-amp is used at the antenna or the Radio is mounted on top of the tower close to the antenna to get rid of all the feed line loss.
This is done by serious VHF/UHF operators to maximize Rx and Tx performance.
.
Throwing around dB figures;
3dB is a loss of half of both the Rx and Tx signals.
This is as simple as it get to understand the whole issue.
7/8"Heliax on 2m and above begins to look good if your serious.
Check the loss per 100' and compare. It's the only way to break Feed line limit from using lesser cable.
Good luck.
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KB1GMX

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Re: VHF/UHF height vs. feedline loss question
« Reply #13 on: May 14, 2022, 08:46:56 AM »

While some have covered gain vs height or more gai in the antenna both are good
to know.

However a different view.

How much of that 150FT of RG213 is unchangeable?  Let say that 70-75ft of it
is going up the tower.  We can easily replace that with much better.  How about
the other end where it is accessible can any amount of that be replaced with
low loss cable.   So some of that 150ft of that RG213 is replaceable.  Lets see
what that means.

At 2M using RG213, its 3.3db, at 440 it will be 7.4DB... this is where your at!

The assumption is half of the 150ft run can be changed out.
FYI properly installed connectors (plus and barrels) add
less than .2db so they are not a real factor.  This is more
important at 432 and up where type N is a good way to go.

Alternate:
RG213 75ft  at 1.9db loss and  75ft of LMR400 at 1.1db  net 3db a
small improvement at 2M.   For the 440 case its 3.7dband 2db for
5.7 total a net 1.7db improvement.

RG213 75ft  at 1.9db loss and  75ft of LMR600 at .7db  net 2.6db a
larger improvement at 2M but really significant at  432/440.
For the 440 case 3.7db + 1.3=5db vs 7.4 a 2.4db improvement just
replacing what goes up the tower.

What if you replace it all, I heard you can't but hang in there.
This is what you get....

Absolute best for the whole 150ft is 1.4db using LMR600 for the whole run.
That's for 2M, at 440 2.25db.  compared to RG213 significant improvement

More reasonable LMR400 case is 2.3db(144) and 3.9db (440) still much better.

As a note at 10M the RG213 is 1.6db loss.  I always argue if a cable
is being installed in cases where changing it later is a problematic
then use the absolute best (Non contaminating RG series) and lowest loss
such as LMR400 or larger, Andrews(hard line), or other direct burial
cable.

So replacing any of the exposed higher loss cable for VHF and up
can have a a marked improvement.  I'd add if your using RG8X or
RG58 for short jumpers that adds up at VHF/UHF while being
mostly insignificant HF.

Allison
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W1VT

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Re: VHF/UHF height vs. feedline loss question
« Reply #14 on: May 14, 2022, 08:55:43 AM »

Lowering the cable loss will increase coverage area, but is that additional area useful?

In New England or California, increased coverage into the "water grids" is rarely useful.
If there is no dry land there is usually no reason to cover that area on 2M/70cm.

If you live on the Midwest you may need to consult local experts.

Large population centers in Connecticut often don't have a lot of hams.
Most of the active hams in central CT live outside the city of Hartford.
« Last Edit: May 14, 2022, 08:59:56 AM by W1VT »
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