# Antenna Length For 800Mhz

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#### kc8syo

##### Member
What length would I need trim off my ant to listen to 866 to 869Mhz? TIA

#### kc8syo

##### Member
Let me clarify my question how long should the whip be? TIA

#### Luis_C

##### Member
Is it a 1/2, 1/4 or a 5/8, what kind of antenna? There's many calculators and formulas on the Internet. Calculate the center of what you what to monitor. I would take 867.5 Mhz in this case.

For example in this website: http://www.csgnetwork.com/antennagpcalc.html

It is for a 1/4 wave antenna. It gives 0.270 feet for example in 867.5.

Hope it helps.

#### DickH

##### Member
kc8syo said:
Let me clarify my question how long should the whip be? TIA

C'mon now. You're a Ham and you don't know that? That's sad.
Anyway, 866-869 freqs. will all change to 851-854 during re-banding..

#### kc8syo

##### Member
Sorry Dick I never said I was a smart Ham LOL. Thanks everyone for the help.

#### DaveH

##### Member
kc8syo said:
Sorry Dick I never said I was a smart Ham LOL. Thanks everyone for the help.

Being "smart" per se is more about asking the right question(s) and giving the
right info. Otherwise it is hard to give a good answer. If it's a regular 1/4 wave
commercial mount, a "cutting chart" should be available. Otherwise just calculate
1/4 wavelength at the frequency; it isn't going to be super-critical.

That said....there is a small correction (K-factor) based on the wavelength
v. diameter of the antenna material. It is ususally only a few percent but I am
sure the commercial vendors take it into account...probably not something

If it's a collinear (multi-section) antenna you're talking about, usually only
the bottom section is trimmed, and again the vendor cutting chart is used
(if you have one). The required length depends on the design.

Dave

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#### Yokoshibu

##### Member
(speed of light 299792458 m / s) / (868000000 freq) = (0.345 meters) * (Velocity factor of coax ??? example fig .89 % ) = (.307 meters) * (5/8 wave) = (0.19211930 meters)

#### Al42

##### Member
Yokoshibu said:
* (Velocity factor of coax ???
Only if you're cutting a piece of coax to be resonant. Cutting an antenna has nothing to do with the VF of any cable connected to it.

#### SCPD

##### QRT
I have one very simlar to ST 2 and wish to CUT it down for Ohio MARCS as an outside antenna. what should I do to sacrifice this to get some reception. I dont care about Multiband reception strickly 700-800 range, any ideas how much to chop and where would be appreciated, Thanks

#### jhooten

##### Member
1/4 wave, 2808/f(MHZ)= length of antenna in inches.

#### ko6jw_2

##### Member
The correct answer is don't cut anything. The difference on a receive and, likely on a transmit antenna will be non-existent. The formula that I use shows a difference of .0113 inches for a 1/4 wave antenna at 866 vs. 869. There are so many other variables that this difference should be ignored.

#### teufler

##### Member
AT THE MOTOROLA Shop I was at, all 800 mhz antennas came out of the packages and nothing was trimmed . I can't remember the actual frequencies but the band width was 50 to 60 mhz at 800 mhz so tuning didn't accomplish much.

#### SCPD

##### QRT
But dont they compromise reception when they say it picks up everything from 39 MHZ to 1300 MHZ, Its one of those grove type antennas like three radials out the top three out the bottom, middle one longe than all. So to cut down to something usable does make sense, but how much should you go to to get a 775-853 range best reception, Thanks

#### ko6jw_2

##### Member
What you are describing sounds like a discone antenna. Discones don't compromise reception. Like log periodics they are flat over a wide frequency range. The only compromise is that discones do not have any gain. Log periodic have gain, but they are directional. Cutting a discone is probably not a good idea unless you know a lot about antenna design and have some test equipment. You could build one yourself. The ARRL Antenna book has design specs. There are also commercial discones made by Telewave, but they are very expensive.

#### SCPD

##### QRT
its an ST 2 style, and I have two left thumbs, at this point all I want to do is hear adequate signals on MARCS with an antenna up in the air. I ws told maybe to cut aerials to ten inches would be in the range, or even an outside car antenna in the cell phone style would grab signals.

#### robbie2745

##### Member
to the OP,

for 1/4 wave, 866-869 MHz would be 3.24-3.23 inches (2808/freq= length in inches)

Can't believe anyone couldn't answer your OP with a simple math equation!

#### profiledescent

##### Member
to the OP,

for 1/4 wave, 866-869 MHz would be 3.24-3.23 inches (2808/freq= length in inches)

Can't believe anyone couldn't answer your OP with a simple math equation!

Congrats on being a straight shooter. I was reading this, and about to do the math and post as well.

To the OP. Don't get to wound around the axel on receive antena length, receive gain, etc. If you are transmitting it's a big deal. Receiving, ....nah.

#### jhooten

##### Member
Can't believe anyone couldn't answer your OP with a simple math equation!

Take a look at post number 10 above.

##### Member
800 antenna

after reading all this just do as i done. if it is a mobile install just cut the end of the cable where it would connect to the antenna expose about 3 inches of the center wire. stick it out the window straight up of course. plug it in to your scanner i done this fo a temp set up an recieved 800 trs about 30 miles away while the vehicle was moving the only problem was the wife didnt like a short wire sticking out the rolled up window also recieved the schp all the way thru the state on a recent trip good temp fix at least fo me

#### mmckenna

##### I ♥ Ø
I have one very simlar to ST 2 and wish to CUT it down for Ohio MARCS as an outside antenna. what should I do to sacrifice this to get some reception. I dont care about Multiband reception strickly 700-800 range, any ideas how much to chop and where would be appreciated, Thanks

If you are having reception issues, there are a number of things that would be more useful to consider before trimming the elements on your antenna.

A couple of things you should really consider first:

How high is the antenna? Higher frequencies work more by line of sight, so if your antenna cannot "see" the transmit site, performance is going to be impacted. The 700 and 800MHz bands are not as forgiving as the lower frequencies, so if your antenna is low to the ground, or well shielded by other structures, this can drastically impact performance. Getting the antenna up higher is going to improve your performance on VHF, UHF and the 700-800MHz bands.

What are you using for coaxial cable, and how long is it? Coaxial cable is the critical link between your antenna and your radio. Using low grade coaxial cable will result in a lot of signal loss between your antenna and radio. The longer the cable, the more loss there will be. Also, the higher you go in frequency, the more pronounced the signal loss is. When you are getting up around 800MHz, you need to look closely at this.

If 700MHz and 800MHz are a primary concern, you might want to consider an antenna designed to work better on these bands. "All band" antennas are not necessarily the best solution. Adding a dedicated antenna, maybe even a directional one aimed at your area of interest, might be a valuable solution.

Trimming the elements on your existing antenna likely isn't going to create a very big improvement.

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