need help with vhf mobile project

Discussion in 'Homebrew and Kit Projects' started by KD8EPC, Aug 20, 2008.

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  1. KD8EPC

    KD8EPC Ham Member QRZ Page

    hello fellow hams i am currently working on making a portable set up for vhf (mobile radio/battery/battery charger/ antenna) I need recommendations for a power source (batterys) i would liuke to be able to put the full power output on the radio (about 40 watts) and would like to be able to charge it while im transmitting without a buzzing sond from a charger/ power supply my main concern is the batterys i need something light weight that can be transported without worrying about spilling acid out of it (like a car battery would do). This is mostly for when i go on vacation or camping but will have limited power most of the time. Any suggestions would be appreciated thanks and 73,

    Dustin KD8EPC
  2. WA9SVD

    WA9SVD Ham Member QRZ Page

    Have you considered a small nuclear reactor? UMM, sorry, you said light weight...:eek:

    Seriously, it WILL depend upon how long you want to operate "portable." Your 40 Watt (sounds like an FM mono or dual-bander) will probably draw about l5 amperes on transmit. (+/- an Ampere or two.) That is continuous draw during transmit. Figure an ampere or two current draw on recenve.

    What you WILL need will be a deep cycle battery If you plan to receive (monitor) continuously, you will need 1-2 Amperes x 24 hours = 24-48 Ampere-Hours.
    Add maybe a 10% period of transmitting, at 15 Amperes:

    15A x 10% x 24 hours = 36 Ampere hours.

    So for continuous operation, you would need at least a 60-84 Ampere hour battery.

    A deep-cycle RV/Marine rated at 90 Ampere hours would likely be adequate, but doesn't meet your criterion of being light weight. (guesstimate 30-50 pounds.) There ARE sealed batteries available in that range, but they aren't inexpensive.
    You woulsd also need a proper charger, but then the question arises, how is the charger powered? If you will be operating for one day or less, and then return to a place where AC is available, then a three state charger may well be adequate, and can recharge the battery in from a few hours to a day, and revert to trickle charge when the battery is fulle charged.
    The only practical alternative would be a solar panel based charging system, that would probably require more time to fully charge the battery, and be more costly, but would be independent of commercial power. (Check the offerings at "Harbor Freight" for solar panels of various sizes; you may need a charge controller with some solar panels, particularly those with higher capacity, to regulate the output and prevent overcharging of a battery.)
  3. KB1LQD

    KB1LQD Ham Member QRZ Page

    That is some good information in the post above. My 2 cents... Inexpensive will lean towards lead-acid deep cycle batteries but the average 17 AH lead acid will weigh 10 pounds or so... More expensive Lithium-Ion will save some weight but will hit the wallet a bit more. The operating activity will definitely effect the systems requirements. If you want to spend a bit more you can find relatively cheap, flexible solar chargers for hiking that can slowly charge your system when in receive (As long as its not drawing too much current on receive!). In this case, doing more RX that TX would help.

    Usually the biggest factor with designing a system such as this is the operating style of the radios. Using efficient radios with lower RX current, as well as using lower power when possible will allow you to use smaller batteries!


    Id like to add in that if your running 12V and putting out 40 Watts, that's just over 3 amps on TX, give about 4 amps to be safe.... As for charging, it is usually very hard to charge a battery while your transmitting because your will need to input more power to the whole system than you are using to operate the radios! If you are using a wall outlet it might be more possible in a semi-portable situation, any charger will do for the most part. As for the hum, if the charger doesn't do output filtering well enough then you can try adding a decent capacitor across the leads of the charger.
    Last edited: Aug 20, 2008
  4. KD8EPC

    KD8EPC Ham Member QRZ Page

    most the time ill be places where i could plug in a charger. Mostly this is for when i go places like camping and youth retreats with my church youth group. Its kinda to get people interested. I took my ht last time i went but it really didn't work out to well so i am going to take a mobile 2 meter rig this year and a quarter wave ground plane. Ive just got to figure out how to power it before i do anything else. Even if i can find a sealed battery thats a little heavy thats fine as long as i can fit it in my bag so i can set it up when i get to where im going.:)
  5. VK2TIL

    VK2TIL Ham Member QRZ Page

    I use a car "jump-starter" for portable power; like this one;

    17 Ah seems to be "standard" for the cheap ones.

    I removed the heavy leads and fitted a cigarette-lighter socket (in one end) and a couple of small co-axial DC sockets (in the holes for the jump-leads).

    It's more bulky than just a battery but it's easy to carry & charge.

    I bought mine cheap from a secondhand shop; there must be a lot of these at garage sales. If buying s/h, check the battery first, though; if it shows 11 or 12 volts it should be OK; if it's very low it might not accept a charge.
  6. K0RGR

    K0RGR Ham Member QRZ Page

    For less money and a lot less hassle, you could build or buy a portable beam that would make your 5 watt HT sound like a 50 watt mobile.

    Arrow makes nice units, and if you have a dual-band HT, you could use it to work the satellites, too. There are tons of homebrew designs, and a 2 meter beam is about the easiest you can build. There are several out there designed for backpacking that break down into very small packages.
  7. ZL3GSL

    ZL3GSL Ham Member QRZ Page

    But I'd guess that the "image" of ham radio needs a "real" tranceiver with a separate microphone.

    The audience see a guy holding a handheld to his mouth and talking to someone, and think: "Great; I can do that with my cellphone, and it takes and transmits photos and I can look at websites, too."

    If you've got power available, a 7Ah gel battery is lighter and you can float it on a small regulated 13.8 supply. The PSU doesn't have to handle the brief transmit current; the battery does that. Otherwise a 17Ah will give you a bit more oomph.

    If you're buying a second-hand battery, a battery below 11V doesn't "need charging"; it doesn't need buying. It's dead.
  8. KD8EPC

    KD8EPC Ham Member QRZ Page

    thanks for all the advice guys heres what ive got so far i also built an antenna i just gotta figure out how ot power it i was thinking of a couple of sealed batteries like are used in power failure lights as backup and wiring them together then a small battery charger i would like to have a car battery but for my purpose that is way to heavy to get a sealed car battery. right now its attached to a group 24 size battery box which i would like to use as the enclosure for whatever i do sense ive already got it around and set up. Thanks for looking at the post and keep replying
    Last edited: Aug 28, 2008
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